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MC LC

Chng 1 ...............................................................................................................................6 M u ..................................................................................................................................6

1.1 Mng truyn thng cng nghip ...................................................................................6 1.2. Vai tr ca mng truyn thng cng nghip ................................................................7 1.3. Phn loi v c trng cc h thng MCN...................................................................8

Chng 2 .............................................................................................................................12 C s k thut......................................................................................................................12

2.1. Cc khi nim c bn ................................................................................................12 2.1.1. Thng tin, d liu v tn hiu..............................................................................12 2.1.2. Truyn thng, truyn d liu v truyn tn hiu ..................................................14 2.1.3. Tnh nng thi gian thc.....................................................................................18

2.2. Ch truyn ti .......................................................................................................18 2.2.1. Truyn bit song song v truyn bit ni tip .........................................................19 2.2.2 Truyn ng b v khng ng b ......................................................................20 2.2.3. Truyn mt chiu v truyn hai chiu.................................................................20 2.2.4 Truyn ti di c s, di mang v di rng ..........................................................21

2.3. Cu trc mng Topology ........................................................................................23 2.3.1. Cu trc bus .......................................................................................................24 2.3.2. Cu trc mch vng (tch cc)............................................................................25 2.3.3. Cu trc hnh sao (hnh 2.12)..............................................................................27 2.3.4 Cu trc cy ........................................................................................................28

2.4. Kin trc giao thc....................................................................................................28 2.4.1. Dch v truyn thng ..........................................................................................28 2.4.2 Giao thc.............................................................................................................30 2.4.3. M hnh lp........................................................................................................32 2.4.4. Kin trc giao thc OSI......................................................................................34 2.4.6 Chun MMS (Manufacturing Message Specification) .........................................41

2.5. Truy nhp bus ...........................................................................................................44 2.5.1. t vn ..........................................................................................................44 2.5.2. Ch/t (Master/Slave) ........................................................................................47 2.5.3. TDMA................................................................................................................48 2.5.4. Token Passing ....................................................................................................49 2.5.5. CSMA/CD..........................................................................................................51 2.5.6. CSMA/CA..........................................................................................................53

2.6. Bo ton d liu ........................................................................................................55

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2.6.1. t vn ..........................................................................................................55 2.6.2. Bit chn l (Parity bit) ........................................................................................57 2.6.3. Bit chn l 2 chiu..............................................................................................58 2.6.4. CRC ...................................................................................................................59 2.6.5. Nhi bit (Bit Stuffing) ........................................................................................61

2.7. M ha bit .................................................................................................................62 2.7.1. Cc tiu chun trong m ha bit..........................................................................62 2.7.2. NRZ, RZ ............................................................................................................64 2.7.3. M Manchester...................................................................................................64 2.7.4. AFP....................................................................................................................65 2.7.5. FSK....................................................................................................................65

2.8. K thut truyn dn...................................................................................................66 2.8.2. RS-232 ...............................................................................................................69 2.8.3. RS-422 ...............................................................................................................71 2.8.4. RS-485 ...............................................................................................................72 2.8.5. MBP (IEC 1153-2) .............................................................................................78

Chng 3 .............................................................................................................................80 Cc thnh phn h thng mng ............................................................................................80

3.1 Phng tin truyn dn ..............................................................................................80 3.1.1 i dy xon .......................................................................................................81 3.1.2 Cp ng trc .....................................................................................................83 3.1.3 Cp quang ...........................................................................................................84 3.1.4 V tuyn..............................................................................................................85

3.2. Giao din mng .........................................................................................................86 3.2.1 Cu trc chung cc phn cng giao din mng ...................................................87 3.2.2. Ghp ni PLC...................................................................................................89 3.2.3 Ghp ni PC........................................................................................................90 3.2.4 Ghp ni vo/ra phn tn .....................................................................................92 3.2.5 Ghp ni cc thit b trng ................................................................................92

3.3. Phn mn trong h thng mng .................................................................................93 3.3.1. Phn mm giao thc ...........................................................................................93 3.3.2. Phn mm giao din lp trnh ng dng..............................................................94

3.4. Thit b lin kt mng................................................................................................95 3.4.1 B lp..................................................................................................................96 3.4.2. Cu ni ..............................................................................................................97 3.4.3. Router ...............................................................................................................97 3.4.4 Gateway ..............................................................................................................98

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3.5. Cc linh kin mng khc ...........................................................................................99 Chng 4 ...........................................................................................................................101 Cc h thng bus tiu biu .................................................................................................101

4.1. PROFIBUS ............................................................................................................101 4.1.1 Kin thc giao thc............................................................................................101 4.1.2 Cu trc mng v k thut truyn dn................................................................102 4.1.3 Truy nhp bus....................................................................................................104 4.1.4 Dch v truyn d liu ........................................................................................105 4.1.5 Cu trc bc in...............................................................................................107 4.1.6 PROFIBUS - FMS ............................................................................................108 4.1.7 PROFIBUS - DP ...............................................................................................113 4.1.8 PROFIBUS - PA ...............................................................................................119

4.2 CAN.........................................................................................................................121 4.2.1 Kin trc giao thc ............................................................................................121 4.2.2 Cu trc mng v k thut truyn dn................................................................122 4.2.3 C ch giao tip.................................................................................................123 4.2.4 Cu trc bc in ..............................................................................................123 4.2.5 Truy nhp bus....................................................................................................127 4.2.6 Bo ton d liu ................................................................................................127 4.2.7. M ho bit ........................................................................................................128 4.2.8 Cc h thng tiu biu da trn CAN ................................................................128

4.3 DeviceNet ................................................................................................................130 4.3.1 C ch giao tip.................................................................................................131 4.3.2 M hnh i tng.............................................................................................131 4.3.3 M hnh a ch .................................................................................................132 4.3.4 Cu trc bc in ..............................................................................................133 4.3.5 Dch v thng bo .............................................................................................133

4.4 Modbus.....................................................................................................................137 4.4.1. C ch giao tip................................................................................................138 4.4.2 Ch truyn ....................................................................................................139 4.4.3 Cu trc bc in ..............................................................................................140 4.4.4 Bo ton d liu ................................................................................................143 4.4.5 Modbus Plus......................................................................................................144

4.5 Interbus ....................................................................................................................146 4.5 .1 Kin trc giao thc ...........................................................................................146 4.5.2 Cu trc mng v k thut truyn dn................................................................147 4.5.3 C ch giao tip.................................................................................................149

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4.5.4 Cu trc bc in ..............................................................................................151 4.5.5 Dch v giao tip ...............................................................................................152

4.6 AS - i .......................................................................................................................153 4.6.1 Kin trc giao thc ............................................................................................154 4.6.2 Cu trc mng v cp truyn ..............................................................................155 4.6.3 C ch giao tip.................................................................................................156 4.6.4 Cu trc bc in ..............................................................................................156 4.6.5 M ho bit .........................................................................................................157 4.6.6 Bo ton d liu ................................................................................................159

4.7 Foundation Fieldbus.................................................................................................159 4.7.1 Kin thc giao thc ...........................................................................................160 4.7.2 Cu trc mng v k thut truyn dn................................................................161 4.7.3 C ch giao tip.................................................................................................163 4.7.4 Cu trc bc in ..............................................................................................164 4.7.5 Dch v giao tip ...............................................................................................165 4.7.6 Khi chc nng ng dng..................................................................................167

4.8 Ethernet....................................................................................................................169 4.8.1 Kin trc giao thc ............................................................................................169 4.8.2 Cu trc mng v k thut truyn dn................................................................170 4.8.3 C ch giao tip.................................................................................................172 4.8.4 Cu trc bc in...............................................................................................172 4.8.5 Truy nhp bus....................................................................................................174 4.8.6 Hiu sut ng truyn v tnh nng thi gian thc...........................................174 4.8.7 Mng LAN 802.3 chuyn mch.........................................................................175 4.8.8 Fast Ethernet .....................................................................................................176 4.8.9 High speed Ethernet ...........................................................................................177 4.8.10 Industrial Ethernet ...........................................................................................179

Chng 5 ...........................................................................................................................180 Mt s vn trong tch hp h thng ...............................................................................180

5.1 Thit k h thng mng ............................................................................................180 5.1.1 Phn tch yu cu...............................................................................................180 5.1.2 Cc bc tin hnh ............................................................................................181

5.2. nh gi v la chn gii php mng......................................................................182 5.2.1 . c th ca cp ng dng................................................................................182 5.2.2. c th ca lnh vc ng dng..........................................................................183 5.2.3. Yu cu thit k chi tit ....................................................................................185 5.2.4. Yu cu kinh t.................................................................................................185

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5.3. Mt s chun phn mm tch hp h thng .............................................................186 5.3.1. Chun IEC 61131 - 5........................................................................................186 5.3.2. OPC (OLE for Process Control) .......................................................................188

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Chng 1 M u

1.1 Mng truyn thng cng nghip Mng truyn thng cng nghip hay mng cng nghip l mt khi nim chung ch cc h thng mng truyn thng s, truyn bit ni tip, c s dng ghp ni cc thit b cng nghip. Cc h thng truyn thng cng nghip ph bin hin nay cho php lin kt mng nhiu mc khc nhau, t cc cm bin, c cu chp hnh di cp trng cho n cc my tnh iu khin, thit b quan st, my tnh iu khin gim st v cc my tnh cp iu hnh x nghip, qun l cng ty. thy r phm vi cp ca lnh vc truyn thng cng nghip, ta cn phn bit vi cc h thng mng vin thng v mng my tnh. C th: + Mng vin thng c phm vi a l v s lng thnh vin tham gia ln hn rt nhiu, nn cc yu cu k thut (cu trc mng, tc truyn thng, tnh nng thi gian thc,) rt khc, cng nh cc phng php truyn thng (truyn ti di rng, iu bin, dn knh, chuyn mch,) thng phc tp hn nhiu so vi mng cng nghip. + i tng ca mng vin thng bao gm c con ngi v thit b k thut, trong con ngi ng vai tr ch yu. V vy cc dng thng tin cn trao i bao gm c ting ni, hnh nh, vn bn v d liu. i tng ca mng cng nghip thun ty l cc thit b cng nghip, nn dng thng tin c quan tm duy nht l d liu. Cc k thut v cng ngh c dng trong mng vin thng rt phong ph, trong khi k thut truyn d liu theo ch bit ni tip l c trng ca mng cng nghip. Mng truyn thng cng nghip thc cht l mt dng c bit ca mng my tnh, c th so snh vi mng my tnh thng thng nh sau: + K thut truyn thng s hay truyn d liu l c trng chung ca c hai lnh vc. + Trong nhiu trng hp, mng my tnh s dng trong cng nghip c coi l mt phn ( cc cp iu khin gim st, iu hnh sn xut v qun l cng ty) trong m hnh phn cp ca mng cng nghip. + Yu cu v tnh nng thi gian thc, tin cy v kh nng tng thch trong mi trng cng nghip ca mng truyn thng cng nghip cao hn so vi mt mng my tnh thng thng, trong khi mng my tnh thng i hi cao hn v bo mt. + Mng my tnh c phm vi tri rng rt khc nhau, v d c th nh nh mng LAN cho mt nhm vi my tnh, hoc rt ln nh mng Internet. Trong nhiu trng hp, mng my tnh gin tip s dng dch v truyn d liu ca mng vin thng. Trong khi , cho n nay

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cc h thng mng cng nghip thng c tnh cht c lp, phm vi hot ng tng i hp. S khc nhau trong phm vi v mc ch s dng gia cc h thng mng truyn thng cng nghip vi cc h thng mng vin thng v mng my tnh dn n s khc nhau trong cc yu cu v mt k thut cng nh kinh t. V d, do yu cu kt ni nhiu nn my tnh khc nhau v cho nhiu phm vi ng dng khc nhau, kin trc giao thc ca cc mng my tnh ph thng thng phc tp hn so vi kin trc giao thc cc mng cng nghip. i vi cc h thng truyn thng cng nghip, c bit l cc cp di th cc yu cu v tnh nng thi gian thc, kh nng thc hin n gin, gi thnh h li lun c t ra hng u. 1.2. Vai tr ca mng truyn thng cng nghip Ghp ni thit b, trao i thng tin l mt trong nhng vn c bn trong bt c mt gii php t ng ha no. Mt b iu khin cn c ghp ni vi cc cm bin v c cu chp hnh. Gia cc b iu khin trong mt h thng iu khin phn tn cng cn trao i thng tin vi nhau phi hp thc hin iu khin c qu trnh sn xut. mt cp cao hn, cc trm vn hnh trong trung tm iu khin cng cn c ghp ni v giao tip vi cc b iu khin c th theo di, gim st ton b qu trnh sn xut v h thng iu khin. Vy mng truyn thng cng nghip c vai tr quan trng nh th no trong cc lnh vc o lng, iu khin v t ng ha ngy nay? S dng mng truyn thng cng nghip, c bit l bus trng thay th cch ni im-im c in gia cc thit b cng nghip nh c minh ha di y mang li hng lot nhng li ch c th:

+ n gin ha cu trc lin kt gia cc thit b cng nghip: Mt s lng ln cc thit b thuc cc chng loi khc nhau c ghp ni vi nhau thng qua mt ng truyn duy nht. + Tit kim dy ni v cng thit k, lp t h thng: Nh cu trc n gin, vic thit k h thng tr nn d dng hn nhiu. Mt s lng ln cp truyn c thay th bng mt ng duy nht, gim chi ph ng k cho nguyn vt liu v cng lp t. + Nng cao tin cy v chnh xc ca thng tin: Khi dng phng php truyn tn hiu tng t c in, tc ng ca nhiu d lm thay i ni dung thng tin v cc thit b khng c cch no nhn bit. Nh k thut truyn thng s, khng nhng thng tin truyn i kh b sai lch hn, m cc thit b ni mng cn c thm kh nng t pht hin li v chn on li

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nu c. Hn th na, vic b qua nhiu ln chuyn i qua li tng t - s v s - tng t nng cao chnh xc ca thng tin. + Nng cao linh hot, tnh nng m ca h thng: Mt h thng mng chun ha quc t to iu kin cho vic s dng cc thit b ca nhiu hng khc nhau. Vic thay th thit b, nng cp v m rng phm vi chc nng ca h thng cng d dng hn nhiu. Kh nng tng tc gia cc thnh phn (phn cng v phn mm) c nng cao nh cc giao din chun. + n gin ha vic tham s ha, chn on, nh v li, s c ca cc thit b: Vi mt ng truyn duy nht, khng nhng cc thit b c th trao i d liu qu trnh, m cn c th gi cho nhau cc d liu tham s, d liu trng thi, m cn c th gi cho nhau cc d liu tham s, d liu trng thi, d liu cnh bo v d liu chn on. Cc thit b c th tch hp kh nng t chn on, cc trm trong mng cng c th c kh nng cnh gii ln nhau. Vic cu hnh h thng, lp trnh, tham s ha, chnh nh thit b v a vo vn hnh c th thc hin t xa qua mt trm k thut trung tm. + M ra nhiu chc nng v kh nng ng dng mi ca h thng: S dng mng truyn thng cng nghip cho php p dng cc kin trc iu khin mi nh iu khin phn tn, iu khin phn tn vi cc thit b trng, iu khin gim st hoc chn on li t xa qua Internet, tch hp thng tin ca h thng iu khin v gim st vi thng tin iu hnh sn xut v qun l cng ty. u th ca gii php dng mng cng nghip khng nhng nm phng din k thut, m cn kha cnh hiu qu kinh t. Chnh v vy, ng dng ca n rng ri trong hu ht cc linh vc cng nghip, nh iu khin qu trnh, t ng ha x nghip, t ng ha ta nh, iu khin giao thng,Trong iu khin qu trnh, cc h thng bus trng dn thay th cc mch dng tng t (current loop) 4-20mA. Trong cc h thng t ng ha x nghip hoc t ng ha ta nh, mt s lng ln cc phn t trung gian c b qua nh cc h bus ghp ni trc tip cc thit b cm bin v chp hnh. 1.3. Phn loi v c trng cc h thng MCN sp xp, phn loi v phn tch c trng cc h thng mng truyn thng cng nghip, ta da vo m hnh phn cp quen thuc cho cc cng ty, x nghip sn xut. Vi loi m hnh ny, cc chc nng c phn thnh nhiu cp khc nhau, nh c minh ha sau y:

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M hnh phn cp chc nng cng ty sn xut cng nghip + Bus trng, bus thit b: Bus trng (fieldbus) l mt khi nim chung c dng trong cc ngnh cng nghip ch bin ch cc h thng bus ni tip, s dng k thut truyn tin s kt ni cc thit b thuc cp iu khin (PC, PLC) vi nhau v vi cc thit b cp chp hnh, hay cc thit b trng. Cc chc nng chnh ca cp chp hnh l o lng, truyn ng v chuyn i tn hiu trong trng hp cn thit. Cc thit b c kh nng ni mng l cc vo/ra phn tn (distributed I/O), cc thit b o lng (sensor, transducer, transmitter) hoc c cu chp hnh (actuator, valve) c tch hp kh nng x l truyn thng. Mt s kiu bus trng ch thch hp ni mng cc thit b cm bin v c cu chp hnh vi cc b iu khin cng c gi l bus chp hnh/cm bin. Trong cng nghip ch to (t ng ha dy chuyn sn xut, gia cng, lp rp) hoc mt s lnh vc ng dng khc nh t ng ha ta nh, sn xut xe hi, khi nim bus thit b li c s dng ph bin. C th ni bus thit b v bus trng c chc nng tng ng nhng do nhng c trng ring bit ca hai ngnh cng nghip, nn mt s tnh nng cng khc nhau. Tuy nhin s khc nhau ny ngy cng tr nn khng r rt, khi m phm vi ng dng ca c hai loi u c m rng v an cho sang nhau. Trong thc t ngi ta cng dng chung mt khi nim l bus trng. Do nhim v ca bus trng l chuyn d liu qu trnh ln cp iu khin x l v chuyn quyt nh iu khin xung cc c cu chp hnh, v vy yu cu v tnh nng thi gian thc c t ln hng u. Thi gian phn ng tiu biu nm trong phm vi t 0.1 ti vi miligiy. Trong khi , yu cu v lng thng tin trong mt bc in thng ch hn ch trong khong mt bi byte, v vy tc truyn thng thng ch cn phm vi Mbit/s hoc thp hn. Vic trao i thng tin v cc bin qu trnh ch yu mang tnh cht nh k, tun hon, bn cnh cc thng tin tham s ha hoc cnh bo c tnh cht bt thng.

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Cc h thng bus trng c s dng rng ri nht hin nay l PROFIBUS, ControlNet, INTERBUS, CAN, WorldFIP, P-NET, Modbus v gn y phi k ti Foundation Fieldbus, DeviceNet, AS-I, EIB v Bitbus l mt vi h thng bus cm bin/chp hnh tiu biu. + Bus h thng, bus iu khin: Cc h thng mng cng nghip c dng kt ni cc my tnh iu khin v cc my tnh trn cp iu khin gim st vi nhau c gi l bus h thng (system bus) hay bus qa trnh (process bus). Qua bus h thng m cc my tnh iu khin c th phi hp hot ng, cung cp d liu qu trnh cho cc trm k thut v trm quan st (c th gin tip thng qua h thng qun l c s d liu trn cc trm ch) cng nh nhn mnh lnh, tham s iu khin t cc trm pha trn. Thng tin khng nhng c trao i theo chiu dc, m cn theo chiu ngang. Cc trm k thut, trm vn hnh v cc trm ch cng trao i d liu qua bus h thng (Trong mt s gii php h thng, mt mng ring gi l terminal bus c s dng ni my ch vi cc trm k thut v trm vn hnh. Tuy nhin y l mt vn thit k gii php, thc ra terminal bus khng c c trng g khc so vi bus h thng). Ngoi ra cc my in bo co v lu tr d liu cng c th c kt ni qua mng ny. S phn bit gia cc khi nim bus trng v bus h thng khng bt buc nm s khc nhau v kiu bus c s dng, m mc ch s dng- hay ni cch khc l thit b c ghp ni. Trong mt s gii php, mt kiu bus duy nht c dng cho c hai cp ny. i vi bus h thng, ty theo lnh vc ng dng m i hi v tnh nng thi gian thc c c t ra mt cch ngt ngho hay khng. Trong thi gian phn ng tiu biu nm trong khong mt vi trm miligiy, trong khi lu lng thng tin cn trao i ln hn nhiu so vi bus trng. Tc truyn thng tiu biu ca bus h thng nm trong phm vi t vi trm Kbit/s n vi Mbit/s. Khi bus h thng c s dng ch ghp ni theo chiu ngang gia cc my tnh iu khin, ngi ta thng dng khi nim bus iu khin. Vai tr ca bus iu khin l phc v trao i d liu thi gian thc gia cc trm iu khin trong mt h thng c cu trc phn tn. Bus iu khin thng thng c tc truyn khng cao, nhng yu cu v tnh nng thi gian thc thng rt kht khe. Do cc yu cu v tc truyn thng v kh nng kt ni d dng nhiu loi my tnh, hu ht cc kiu bus h thng thng dng u da trn nn Ethernet nh Industrial Ethernet, Fieldbus Foundations High Speed Ethernet (HSE), Ethernet/IP. Bn cnh phi k n PROFIBUS-FMS, ControlNet v Modbus Plus. + Mng x nghip: L mt mng LAN bnh thng, c chc nng kt ni cc my tnh vn phng thuc cp iu hnh sn xut vi cp iu khin gim st. Thng tin c a ln trn bao gm trng thi lm vic ca cc qu trnh k thut, cc gin my cng nh ca h thng

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iu khin t ng, cc s liu tnh ton, thng k v din bin qu trnh sn xut v cht lng sn phm. Thng tin theo chiu ngc li l cc thng s thit k, cng thc iu khin v mnh lnh iu hnh. Ngoi ra, thng tin cng c trao i mnh theo chiu ngang gia cc my tnh thuc cp iu hnh sn xut, v d h tr kiu l vic theo nhm, cng tc trong d n, s dng chung cc ti nguyn ni mng nh my in, my ch, Khc vi cc h thng bus cp di, mng x nghip khng yu cu nghim ngt v tnh nng thi gian thc. Vic trao i d liu thng din ra khng nh k, nhng c khi vi s lng ln ti hng Mbyte. Hai loi mng c dng ph bin cho mc ch ny l Ethernet v Token-Ring, trn c s cc giao thc chun nh TCP/IP v IPX/SPX. + Mng cng ty: Mng cng ty nm trn cng trong m hnh phn cp h thng truyn thng ca mt cng ty sn xut cng nghip. c trng ca mng cng ty gn vi mt mng vin thng hoc mt mng my tnh din rng nhiu hn trn cc phng din phm vi v hnh thc dch v, phng php truyn thng v cc yu cu v k thut. Chc nng ca mng cng ty l kt ni cc my tnh vn phng ca cc x nghip, cung cp cc dch v trao i thng tin ni b v vi cc khch hng nh th vin in t, th in t, hi tho t xa qua in thoi, hnh nh, cung cp dch v truy cp Internet v thng mi in t,Hnh thc t chc ghp ni mng cng nh cc cng ngh c p dng rt a dng, ty thuc vo u t ca cng ty. Trong nhiu trng hp, mng cng ty v mng x nghip c thc hin bng mt h thng mng duy nht v mt vt l, nhng chia thnh nhiu phm vi v nhm mng lm vic ring bit. Mng cng ty c vai tr nh mt ng cao tc trong h thng h tng c s truyn thng ca mt cng ty, v vy i hi v tc truyn thng v an ton, tin cy c bit cao. Fast Ethernet, FDDI, ATM l mt vi v d cng ngh tin tin c p dng y trong hin ti v tng lai.

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Chng 2 C s k thut

2.1. Cc khi nim c bn 2.1.1. Thng tin, d liu v tn hiu a. Thng tin: Thng tin l mt trong nhng khi nim c s quan trng nht trong khoa hc k thut, cng ging nh vt cht v nng lng. Cc u vo cng nh cc u ra ca mt h thng k thut ch c th l vt cht, nng lng hoc thng tin c m t nh hnh 2.1 di:

Thng tin l thc o mc nhn thc, s hiu bit v mt vn , mt s kin hoc mt th thng. V d, mt thng tin cho chng ta bit mt cch chnh xc hay tng i v nhit ngoi tri hay mc nc trong b cha. Thng tin gip chng ta phn bit gia cc mt ca mt vn , gia cc trng thi ca mt s vt. Ni mt cch khc, thng tin chnh l s loi tr tnh bt nh. Trong khi vt cht v nng lng l nn tng ca vt l v ha hc, th thng tin chnh l ch th ca tin hc v cng ngh thng tin. Thng tin l c s cho s giao tip. Thng qua vic giao tip m cc i tc c thm hiu bit ln nhau hoc v cng mt vn , mt s kin hoc mt h thng. + D liu: Thng tin l mt i lng kh tru tng v vy cn c biu din di mt hnh thc khc. Kh nng biu din thng tin rt a dng v d qua ch vit, hnh nh, c ch,Dng biu din thng tin ph thuc vo mc ch, tnh cht ca ng dng. c bit, thng tin c th c m t hay ni cch khc l c s lng ha bng d liu c th lu tr v x l trong my tnh. Trong trng hp , ta cng ni rng thng tin c s ha s dng h m nh phn, hay m ha nh phn. Ni trong ng cnh cu trc mt bc in, d liu chnh l phn thng tin hu ch c biu din bng cc bit {1,0}. Tuy trong thc t, cc khi nim nh x l thng tin v x l d liu, truyn ti thng tin v truyn ti d liu hay c dng vi cc ngha tng t, ta cn phn bit r rng gia thng tin v d liu. V d, hai tp d liu khc nhau c th m t cng mt ni dung thng tin. Ngc li, hai tp d liu ging nhau c th mang nhng thng tin khc nhau, ty theo cch m t. Ta c th so snh quan h gia d liu v thng tin vi quan h trong ton hc gia s v ngha s dng ca s. Theo ngha th hai, d liu c hiu l phn biu din thng tin hu dng (thng tin ngun) trong mt bc in. Tuy nhin, cn c vo ng cnh c th m ta khng s nhm ln gia hai cch s dng thut ng ny.

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+ Lng thng tin: Thng tin chnh l s xa b tnh bt nh, v d mt s khng nh v mt s kin c xy ra hay khng, mt cu tr li ng hay sai. Mc ca s xa b tnh bt nh ny hay ni cch khc, gi tr v s hiu bit mt ngun thng tin mang li c gi l lng thng tin. Chnh v d liu l mt dng biu din thng tin c th x l c trong my tnh, nn lng thng tin cng c o bng n v d liu (bit=binary digit). Trong trng hp thng tin v s khng nh ng/sai, r rng ch cn mt bit ghi m 1 hoc 0, hay ni cch khc l lng thng tin bng 1 bit. Vi mt v d khc, biu din hay phn bit mt mu no trong s 16 mu ta cn 4bit, trong 256 mu cn 8bit. biu din hay phn bit mt ch ci trong bng k t c 256 ch ci v k hiu cn 8bit. Mt thng bo cn gi i nh Hello gm c 5 k t s cn 40bit. Ta ni rng, lng thng tin y l 4bit, 8bit, 40bitTa c 1Kbit=1024bit. 1Mbit=1024Kbit v 1Gbit=1024Mbit. + Tn hiu: Vic trao i thng tin (gia ngi v ngi, gia ngi v my) hay d liu (gia my v my) ch c th thc hin c nh tn hiu. C th nh ngha, tn hiu l din bin ca mt i lng vt l cha ng tham s thng tin/d liu v c th truyn dn uc. Theo quan im ton hc th tn hiu c coi l mt hm ca thi gian. Trong cc lnh vc k thut, cc loi tn hiu thng dng l in, quang, kh nn, thy lc v m thanh. Cc tham s sau y thng c dng trc tip, gin tip hay kt hp biu th ni dung thng tin: * Bin (in p, dng,) * Tn s, nhp xung, rng ca xung, sn xung * Pha, v tr xung Khng phn bit tnh cht vt l ca tn hiu (in, quang, kh nn,), ta c th phn loi tn hiu da theo tp hp gi tr ca tham s thng tin hoc da theo din bin thi gian thnh nhng dng sau: * Tng t: Tham s thng tin c th c mt gi tr bt k trong mt khong no . * Ri rc: Tham s thng tin ch c th c mt s gi tr (ri rc) nht nh. Thc ra cc ng biu din tn hiu trn hnh 2.2b,c v d ch c tnh cht minh ha, cc tn hiu thc thng khng c dng l tng nh vy, m bin , pha hay tn s ca chng nm trong nhng khong gi tr lin tc. Vic s dng cc dng tn hiu khc nhau cng l th hin thng tin phc v nhng mc ch s dng khc nhau. Khi cc gi tr tham s thng tin ca mt tn hiu c biu din bng m nh phn th dng tn hiu c bit ny c gi l tn hiu s. Ni mt cch khc, tn hiu s dng truyn ti thng tin c m ha nh phn. Vi tn hiu s, ta ch cn phn bit gia hai trng thi ca tn hiu ng vi cc bit 0 v 1, v vy s hn ch c mt cch hiu qu s sai lch thng tin bi s tc ng ca nhiu.

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Lu s phn bit gia tn hiu tng t vi tn hiu ri rc khng n thun dng tn hiu thc t c dng, m quan trng hn l ngha ca tham s thng tin s dng trong tn hiu. Tham s thng tin khng nht thit phi l mt bin s hc m rt c th mt bin logic. Nh vy nhn vo mt tn hiu ta cha th ni ngay c l tn hiu s hay khng. V d ta hon ton c th s dng mt tn hiu nh trn hnh 2.2a m ha cc bt 1 v 0 ng vi bin ln hoc nh hn khng. Trong trng hp ny, tham s thng tin khng phi l gi tr s hc ca bin , m l gi tr logic ca mnh bin ln hn 0? 2.1.2. Truyn thng, truyn d liu v truyn tn hiu + Giao tip v truyn thng: Giao tip hay truyn thng l mt qu trnh trao i thng tin gia hai ch th vi nhau, c gi l cc i tc giao tip, theo mt phng php c qui nh trc. i tc ny c th iu khin i tc kia, hoc quan st trng thi ca i tc. Cc i tc giao tip c th l ngi hoc h thng k thut tc l cc thit b phn cng (i tc vt l) hoc cc chng trnh phn mm (i tc logic). Trong trng hp sau, khi nim truyn thng thng c s dng thay cho khi nim giao tip. Tuy nhin khi nim giao tip c ngha bao trm hn. Trong h thng truyn thng cng nghip, cc i tc truyn thng thun ty l cc thit b, h thng k thut nn hai thut ng giao tip v truyn thng c s dng vi ngha tng ng. thc hin vic giao tip hay truyn thng ta cn cc tn hiu thch hp, c th l tn hiu tng t hay tn hiu s. S phn bit gia tn hiu v thng tin dn ti s phn bit gia x l tn hiu v x l thng tin, gia truyn tn hiu vi truyn thng. C th s dng cc dng tn hiu rt khc nhau truyn ti mt ngun thng tin, cng nh mt tn hiu c th mang nhiu ngun thng tin khc nhau.

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Trn c s cc dng tn hiu khc nhau, ngi ta c th phn bit cc kiu giao tip nh: giao tip ting ni; giao tip hnh nh; giao tip vn bn; giao tip d liu. Chnh v d liu l mt dng biu din thng tin s dng m nh phn, truyn ti thng tin s dng tn hiu s cng c gi l truyn d liu. C th ni, truyn d liu l phng php truyn thng duy nht gia cc my tnh, tc trong mng my tnh. Ngy nay, k thut s cng c ng dng rng ri trong vic truyn ti ting ni, hnh nh v vn bn, v vy truyn d liu ng vai tr quan trng hng u. S phn bit gia tn hiu s v tn hiu tng t ph thuc vo ngha ca tham s thng tin m tn hiu mang. S phn bit gia phng php truyn tn hiu s dng k thut s (gi tt l truyn tn hiu s) vi cc phng php truyn tn hiu truyn thng cng tng t nh vy. Trong cc h thng truyn thng cng nghip ta ch quan tm ti truyn tn hiu s (truyn d liu). Cc chun giao tip trong cc h thng truyn thng cng nghip cng l cc chun giao tip s. + M ha gii m:

Hnh 2.3. minh ha nguyn tc c bn ca truyn thng. Thng tin cn trao i gia cc i tc c m ha trc khi c mt h thng truyn dn tn hiu chuyn ti pha bn kia. Trong thut ng truyn thng, m ha ch qu trnh bin i ngun thng tin (d liu) cn trao i sang mt chui tn hiu thch hp truyn dn. Qu trnh ny t nht thng bao gm hai bc: M ha ngun v m ha ng truyn. Trong qu trnh m ha ngun, d liu mang thng tin thc dng hay d liu ngun c b sung cc thng tin ph tr cn thit cho vic truyn dn, v d a ch bn gi v bn nhn, kiu d liu, thng tin kim li,D liu trc khi gi i cng c th c phn chia thnh nhiu gi d liu bc in ph hp vi phng php truyn, nn li tng hiu sut ng truyn hoc m ha bo mt. Nh vy, lng thng tin cha ng trong mt tn hiu s nhiu hn lng thng tin thc dng cn truyn ti. Sau khi c m ha ngun, m ha ng truyn l qu trnh to tn hiu tng ng vi cc bit trong gi d liu hay bc in theo mt phng php nht nh ph hp vi ng truyn v k thut truyn. Hnh 2.4 minh ha mt v d m ha ng truyn n gin cc bit 0 c th hin bng mc in p cao v cc bit 1 bng mc in p thp.

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Trong truyn thng cng nghip, m ha ng truyn ng ngha vi m ha bit, bi tn hiu do khu m ha tng bit to ra cng chnh l tn hiu c truyn dn. i vi cc h thng truyn thng khc, qu trnh m ha ng truyn c th bao hm vic iu bin tn hiu v dn knh, cho php truyn cng mt lc nhiu ngun thng tin v truyn tc cao. Vic dn knh c th thc hin theo phng php phn chia tn s, phn chia thi gian hoc phn chia m. Trong mt tn hiu c truyn ti i cn c mt phng php bn nhn phn bit gii hn gia cc bit d liu ni tip nhau, gi l phng php ng b ha. to iu kin thc hin c vic ny mt cch n gin, tn hiu thng c pht theo nhp u n, mi nhp ng vi mt bit, nh c minh ha trn hnh 2.4. Qu trnh ngc li vi m ha l gii m, tc l chuyn i cc tn hiu nhn c thnh dy bit tng ng v sau x l, loi b cc thng tin b sung ti to thng tin ngun. + iu ch v iu bin tn hiu: iu ch v iu bin l hai khi nim c dng vi ngha rt gn nhau. iu ch c hiu l qu trnh to mt tn hiu trc tip mang tham s thng tin, th hin qua bin tn s hoc pha, trong tham s thng tin c th ly mt gi tr bt k. Mt trng hp c bit ca iu ch l khi n c s dng vo mc ch truyn d liu v thm s thng tin ch c th ly hai gi tr logic 1 v 0, ngi ta dng khi nim m ha bit. iu ch cn tm thy ng dng trong cc b chuyn i D/A, cc b to xung (iu ch rng xung, iu ch tn s xung, iu ch v tr xung, iu ch m xung). iu bin ch qu trnh dng tn hiu mang thng tin iu khin, bin i cc tham s thch hp ca mt tn hiu th hai (tn hiu mang). Mc ch c bn ca iu bin l s dng mt tn hiu mang c mt di tn khc thc hin phng php dn knh phn chia tn s, hoc trnh truyn dn di tn c s d b nhiu. i khi ranh gii phn bit gia iu ch v iu bin cng khng hon ton r rng, v vy trong thc t khi nim iu ch thng c dng chung cho c hai trng hp (modulation), tuy nhin ty theo ng cnh m c hiu theo hai ngha khc nhau. + Tc truyn v tc bit: Thi gian cn truyn mt tp d liu v d mt k t, ph thuc vo hai yu t l tc baud v phng php m ha bit. Tc baud c nh ngha l s ln tn hiu thay i gi tr tham s thng tin (v d bin ) trong mt giy v c n v l Baud. Do hu ht cc h thng truyn d liu hot ng theo nhp tun hon, tc baud tng ng vi tn s nhp ca h thng thu pht (i vi nhiu phng php m ha bit, tn hiu khng bt buc phi

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thay i trng thi trong mi nhp v th khi nim tc baud khng hon ton chnh xc) thay vo ngi ta s dng cc khi nim tc truyn hay tc bit. Tc truyn hay tc bit c tnh bng s bit d liu c truyn i trong mt giy, tnh bng bit/s hoc bps (bit per second). Nu tn s nhp c k hiu l f v s bit truyn i trong mt nhp l n, s bit c truyn i trong mt giy s l v=f*n. Nh vy c hai cch tng tc truyn ti l tng tn s nhp hoc tng s bit truyn i trong mt nhp. Nu mi nhp ch c mt bit duy nht c chuyn i th v=f. Nh vy, ch i vi cc phng php m ha bit s dng hai trng thi tn hiu v trng thi tn hiu thay i lun phin sau mi nhp th tc bit mi tng ng vi tc baud hay 1baud tng ng vi 1bit/s. Cn phn bit gia tc truyn thng tin hu ch v tc truyn thng tin tng th. Mt thng tin cn truyn i (thng tin hu ch) s m ha ngun, tc c ng gi v b sung cc thng tin ph tr cn thit cho vic truyn ti (overhead). V vy tc truyn thng tin tng th c th ln hn rt nhiu so vi tc truyn thng tin hu ch, ph thuc vo h thng truyn thng. Thc t, tc truyn thng tin hu ch rt kh xc nh c mt cch chnh xc. + Thi gian bit chu k bit: Trong vic phn tch, nh gi tnh nng thi gian ca mt h thng truyn thng th thi gian bit l mt gi tr hay c dng. Thi gian bit hay chu k bit c nh ngha l thi gian trung bnh cn thit chuyn mt bit, hay chnh bng gi tr nghch o ca tc truyn ti: TB=1/v TB=1/f , trng hp n=1. + Thi gian lan truyn tn hiu: Thi gian lan truyn tn hiu l thi gian cn mt tn hiu pht ra t mt u dy lan truyn ti u dy khc, ph thuc vo chiu di v cu to dy dn. Tc lan truyn tn hiu chnh l tc truyn sng in t. Tuy nhin, trong mi trng kim loi hoc si quang hc, gi tr ny s nh hn tc truyn sng in t hay tc nh sng trong mi trng chn khng. Ta c: TS=l/(k*c) Trong : TS: l thi gian lan truyn tn hiu l: l chiu di dy dn c: l tc nh sng trong chn khng (300.000.000m/s) k: Biu th h s gim tc truyn, c tnh theo cng thc: trong l hng s in mi ca lp cch ly i vi cc loi cp c lp bc cch ly l Polyethylen vi hng s in mi , ta c h s . H s ny cng ng vi mi trng truyn cp l cp quang v thng c dng mt cch tng qut tnh ton gi tr tng i ca thi gian lan truyn tn hiu trong nhiu php nh gi. Nh vy TS s ch cn ph thuc vo chiu di dy dn: TS(giy) = l (mt)/200.000.000

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Ch rng thi gian lan truyn tn hiu khng c quan h trc tip vi tc truyn thng. Tuy nhin tnh nng thi gian ca mt h thng truyn thng ph thuc c vo hai thng s ny, trong khi mt s phng php truyn thng i hi s trao i rng buc gia chng. V d, ta khng th ng thi tng chiu di dy dn v tc truyn thng mt cch ty . 2.1.3. Tnh nng thi gian thc Tnh nng thi gian thc l mt trong nhng c trng quan trng nht i vi cc h thng t ng ha ni chung v cc h thng bus trng ni ring. S hot ng bnh thng ca mt h thng k thut lm vic trong thi gian thc khng ch ph thuc vo chnh xc, ng n ca cc kt qu u ra, m cn ph thuc vo thi im a ra kt qu. Mt h thng c tnh nng thi gian thc khng nht thit phi c phn ng tht nhanh, m quan trng hn l phi c phn ng kp thi i vi cc yu cu, tc ng bn ngoi. Nh vy mt h thng truyn thng c tnh nng thi gian thc phi c kh nng truyn ti thng tin mt cch tin cy v kp thi vi yu cu ca cc i tc truyn thng. Tnh nng thi gian thc ca mt h thng iu khin phn tn ph thuc rt nhiu vo h thng bus trng c dng. m bo tnh nng thi gian thc, mt h thng bus phi c nhng c im sau y: + nhanh nhy: Tc truyn thng hu ch phi nhanh p ng nhu cu trao i d liu trong mt gii php c th. + Tnh tin nh: D on trc c v thi gian phn ng tiu biu v thi gian phn ng chm nht vi yu cu ca tng trm. + tin cy, kp thi: m bo tng thi gian cn cho vic vn chuyn d liu mt cch tin cy gia cc trm nm trong mt khong xc nh. + Tnh bn vng: C kh nng x l s c mt cch thch hp khng gy hi thm cho ton b h thng. R rng, kh nng tha mn yu cu v thi gian thc ph thuc vo bi ton ng dng c th. Mt mng cng nghip c tnh nng thi gian thc khng c ngha l s thch ng vi mi ng dng i hi yu cu v thi gian thc. Nhim v ca ngi tch hp h thng l phi la chn v thit k mt gii php thch hp tha mn yu cu ny trn c s phn tch cc tnh nng k thut lin quan di iu kin rng buc l gi thnh chi ph. 2.2. Ch truyn ti Ch truyn ti c hiu l phng thc cc bit d liu c chuyn gia cc i tc truyn thng. Nhn nhn t cc gc khc nhau ta c th phn bit cc ch truyn ti nh sau: + Truyn bit song song hoc truyn bit ni tip + Truyn ng b hoc khng ng b + Truyn mt chiu hay n cng (simplex), hai chiu ton phn, hai chiu ng thi hay song cng (duplex, full-duplex) hoc hai chiu gin on hay bn song cng (half-duplex).

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+ Truyn ti di c s, truyn ti di mang v truyn ti di rng. 2.2.1. Truyn bit song song v truyn bit ni tip + Truyn bit song song: Phng php truyn bit song song (hnh 2.5a) c dng ph bin trong cc bus ni b ca my tnh nh bus a ch, bus d liu v bus iu khin. Tc truyn ti ph thuc vo s cc knh dn, hay cng chnh l rng ca mt bus song song, v d 8bits, 16bits, 32bits hay 64bits. Chnh v nhiu bit c truyn i ng thi, vn ng b ha ti ni pht v ni nhn tn hiu phi c gii quyt. iu ny gy tr ngi ln khi khong cch gia cc i tc truyn thng tng ln. Ngoi ra, gi thnh cho cc bus song song cng l mt yu t dn n phm vi ng dng ca phng php truyn ny ch hn ch khong cch nh, c yu cu rt cao v thi gian v tc truyn.

+ Truyn bt ni tip: Vi phng php truyn bit ni tip, tn bit c chuyn i mt cch tun t qua mt ng truyn duy nht (hnh 2.5b). Tuy tc bit b hn ch nhng cch thc hin li n gin, tin cy ca d liu cao. Tt c cc mng truyn thng cng nghip u s dng phng php truyn ny. Mt mng truyn thng cng nghip c nhim v kt ni cc thit b k thut c kh nng x l thng tin hay ni cch khc l x l d liu. Nhng thit b d tn ti di dng ny hay dng khc cng u l nhng my tnh, c b vi x l v h thng bus ni b song song. V vy, c th dng phng php truyn ni tip, ta cn cc b chuyn i gia bus song song v ni tip, nh c minh ha trn hnh 2.6.

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2.2.2 Truyn ng b v khng ng b S phn bit gia ch truyn ng b v khng ng b ch lin quan ti phng thc truyn bit ni tip. Vn ng b ha gia bn gi v bn nhn d liu li c t ra, lm th no bn nhn bit khi no mt tn hiu trn ng truyn mang d liu gi v khi no khng? Trong ch truyn ng b, cc i tc truyn thng lm vic theo cng mt nhp, tc vi cng tn s v lch pha c nh. C th qui nh mt trm c vai tr to nhp v dng mt ng dy ring mang nhp ng b cho cc trm khc. Bin php kinh t hn l dng mt phng php m ha bit thch hp bn nhn c th ti to nhp ng b t chnh tn hiu mang d liu. Nu phng php m ha bit khng cho php nh vy, th c th dng k thut ng gi d liu v b sung mt dy bit mang thng tin ng b ha vo phn u mi gi d liu. Bn gi v bn nhn ch cn hot ng ng b trong khi trao i d liu. Vi ch truyn khng ng b, bn gi v bn nhn khng lm vic theo mt nhp chung. D liu trao i thng c chia thnh tng nhm 7 hoc 8bit, gi l k t. Cc k t c chuyn i vo nhng thi im khng ng u, v vy cn thm hai bit nh du khi u v kt thc cho mi k t. Vic ng b ha c thc hin vi tng k t. V d, cc mch UART (Universal Asynchronous Receiver/Transmiter) thng dng dng bc in 11 bits bao gm 8 bit k t, 2 bit khi u cng nh kt thc v 1bit kim tra li chn l. 2.2.3. Truyn mt chiu v truyn hai chiu Tng t nh cc ng giao thng, mt ng truyn d liu c kh nng hoc lm vic di ch mt chiu, hai chiu ton phn hoc hai chiu gin on nh hnh 2.7 sau:

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Ch truyn ny t ph thuc vo tnh cht vt l ca mi trng truyn dn, m ph thuc vo phng php truyn dn tn hiu, chun truyn dn (RS-232, RS-422, RS-485,) v vo cu hnh ca h thng truyn dn. Trong ch truyn mt chiu, thng tin ch c chuyn i theo mt chiu, mt trm ch c th ng vai tr hoc bn pht (transmitter) hoc bn nhn thng tin (receiver) trong sut qu trnh giao tip. V d ch truyn ny c s dng nh giao din gia bn phm, chut hoc mn hnh vi my tnh, cc h thng pht thanh truyn hnh,..Ch truyn mt chiu hu nh khng c vai tr i vi mng cng nghip. Ch truyn hai chiu gin on cho php mi trm c th tham gia gi hoc nhn thng tin, nhng khng cng mt lc. Nh vy thng tin c trao i theo c hai chiu lun phin trn cng mt ng truyn vt l. Mt u im ca ch ny l khng i hi cu hnh h thng phc tp lm, trong khi c th t c tc truyn tng i cao. Mt trm c c mt b pht v mt b thu; thut ng nhn to transceiver c hnh thnh t hai ch transmitter v receiver ghp li. Trong khi b pht lm vic th b thu phi ngh v ngc li. Do c tnh ny, ch truyn hai chiu gin on thch hp vi kiu lin kt im nhiu im cng nh kiu nhiu im, hay ni cch khc l thch hp i vi cu trc bus. Trong mt h thng bus, trm no cng c quyn pht nn cn mt phng php phn chia thi gian tc phng php truy nhp bus trnh xung t tn hiu. Trong khi m trm pht th tt c cc trm khc phi c gi trong trng thi thu nhn tn hiu. Ch truyn ny c s dng ph bin trong mng cng nghip, v d vi chun RS-485. Vi ch truyn hai chiu ton phn mi trm u c th gi v nhn thng tin cng mt lc. Thc cht, ch ny ch khc vi ch hai chiu gin on ch phi s dng hai ng truyn ring bit cho thu v pht, tc l khc cu hnh h thng truyn thng. D dng nhn thy, ch truyn hai chiu ton phn ch thch hp vi kiu lin kt im im hay ni cch khc l ph hp vi cu trc mch vng v cu trc hnh sao. 2.2.4 Truyn ti di c s, di mang v di rng + Truyn ti di c s:

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Mt tn hiu mang mt ngun thng tin c th biu din bng tng ca nhiu dao ng c tn s khc nhau nm trong mt phm vi hp, c gi l di tn c s hay di hp. Tn hiu c truyn i cng chnh l tn hiu c to ra sau khi m ha bit, nn c tn s c nh hoc nm trong mt khong hp no , ty thuc vo phng php m ha bit. V d c th qui nh mc tn hiu cao ng vi bit 0 v mc tn hiu thp ng vi bit 1. Tn s ca tn hiu thng nh hn hoc cng lm l tng ng vi tn s nhp bus. Tuy nhin, trong mt nhp (c th tng ng hoc khng tng ng vi chu k ca tn hiu), ch c th truyn i mt bit duy nht, mi thnh vin trong mng phi phn chia thi gian s dng ng truyn. Tc truyn ti v th tuy c b hn ch, nhng phng php ny d thc hin v tin cy, c dng ch yu trong cc h thng mng truyn thng cng nghip. + Truyn ti di mang: Trong mt s trng hp, di tn c s khng tng thch trong mi trng lm vic. V d, tn hiu c cc tn s ny c th bc x nhiu nh hng ti hot ng ca cc thit b in t khc, hoc ngc li, b cc thit b khc gy nhiu. khc phc tnh trng ny, ngi ta s dng mt tn hiu khc gi l tn hiu mang, c tn s nm trong mt di tn thch hp gi l di mang. Di tn ny thng ln hn nhiu so vi tn s nhp. D liu cn truyn ti s dng iu ch tn s, bin hoc pha ca tn hiu mang. Bn nhn s thc hin qu trnh gii iu ch hi phc thng tin ngun. Khc vi truyn ti di rng, phng thc truyn ti di mang ch p dng cho mt knh truyn tin duy nht ging nh truyn ti di c s.

+ Truyn ti di rng: Mt tn hiu c th cha ng nhiu ngun thng tin khc nhau bng cch s dng kt hp mt cch thng minh nhiu thng s thng tin. V d mt tn hiu phc tp c th l tng hp bng phng php xp chng t nhiu tn hiu thnh phn c tn s khc nhau mang cc ngun thng tin khc nhau. Sau khi nhiu ngun thng tin khc nhau c m ha bit, mi tn hiu c to ra s dng iu bin mt tn hiu khc, thng c tn s ln hn nhiu, gi l tn hiu mang. Cc tn hiu mang c iu bin c tn s khc nhau, nn c th pha trn xp chng thnh mt tn hiu duy nht c ph tn tri rng. Tn hiu ny cui cng li c dng iu bin mt tn hiu mang khc. Tn hiu thu c t khu ny mi c truyn i. y chnh l k thut dn knh phn tn trong truyn ti thng tin, nhm mc ch s dng hiu qu hn ng truyn. Pha bn nhn s thc hin vic gii iu bin v phn knh, hi phc cc tn hiu mang cc ngun thng tin khc nhau. Phng thc truyn ti di rng v k thut dn knh c dng rng ri trong cc mng vin thng bi tc cao v kh nng truyn song song nhiu ngun thng tin. Tuy nhin, v c im phm vi mng, l do gi thnh thc hin v tnh nng thi gian, truyn ti bng

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rng cng nh k thut dn knh hu nh khng ng vai tr g trong cc h thng truyn thng cng nghip. 2.3. Cu trc mng Topology Cu trc mng lin quan ti t chc v phng thc phi hp hot ng gia cc thnh phn trong mt h thng mng. Cu trc mng nh hng ti nhiu tnh nng k thut, trong c tin cy ca h thng. Trc khi tm hiu v cc cu trc thng dng trong mng truyn thng cng nghip ta cn nm c mt vi khi nim c bn sau: + Lin kt: Lin kt (link) l mi quan h vt l hoc logic gia hai hoc nhiu i tc truyn thng. i vi lin kt vt l, cc i tc chnh l cc trm truyn thng c lin kt vi nhau qua mt mi trng vt l. V d cc th ni mng trong my tnh iu khin, cc b x l truyn thng ca PLC hoc cc b lp u l cc i tc vt l. Trong trng hp ny, tng ng vi mt nt mng ch c mt i tc duy nht. Khi nim lin kt logic c th c hiu theo hai ngha. - Th nht, mt i tc truyn thng khng nht thit phi l mt thit b phn cng, m c th l mt chng trnh h thng hay mt chng trnh ng dng trn mt trm, nn quan h gia cc i tc ny ch mang tnh cht logic. Nh vy, tng ng vi mt i tc vt l thng c nhiu i tc logic, cng nh nhiu mi lin kt logic c xy dng trn c s mt mi lin kt vt l. - Th hai, mc d bn thn cc i tc vn l cc thit b phn cng, nhng quan h ca chng v mt logic hon ton khc vi quan h v mt vt l. C th phn bit cc kiu lin kt sau y: - Lin kt im im (point-to-point): Mt mi lin kt ch c hai i tc tham gia. Nu xt v mt vt l th vi mt ng truyn ch ni c hai trm vi nhau. xy dng mt mng truyn thng trn c s ny s cn nhiu ng truyn ring bit. - Lin kt im nhiu im (multi-drop): Trong mt mi lin kt c nhiu i tc tham gia, tuy nhin ch mt i tc c nh duy nht (trm ch) c kh nng pht trong khi nhiu i tc cn li (cc trm t) thu nhn thng tin cng mt lc. Vic giao tip theo chiu ngc li t trm t ti trm ch ch c thc hin theo kiu im im. Xt v mt vt l, nhiu i tc c th c ni vi nhau qua mt cp chung duy nht. - Lin kt nhiu im (multipoint): Trong mt mi lin kt c nhiu i tc tham gia v c th trao i thng tin qua li t do theo bt k hng no. Bt c mt i tc no cng c quyn pht v bt c trm no cng nghe c. Cng nh kiu lin kt im nhiu im, c th s dng mt cp dn duy nht ni mng gia cc i tc. Mt h thng truyn thng khng nht thit phi h tr tt c cc kiu lin kt nh trn. ng nhin, kh nng lin kt im nhiu im bao hm kh nng lin kt im im

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cng nh lin kt nhiu im bao hm hai kh nng cn li. Kh nng lin kt nhiu im l c trng ca mng truyn thng cng nghip. + Topology: Topology l cu trc lin kt ca mt mng, hay ni cch khc chnh l tng hp ca cc lin kt. Topology c th hiu l cch sp xp, t chc v mt vt l ca mng, nhng cng c th l cch sp xp logic ca cc nt mng, cch nh ngha v t chc logic cc mi lin kt gia cc nt mng. Hai khi nim topology v cu trc mng khng hon ton ging nhau nhng trong thc t chng c dng vi ngha tng ng. Cc dng cu trc mng c bn l bus, mch vng (tch cc) v hnh sao. Mt s cu trc phc tp hn nh cu trc cy u c th xy dng trn c s phi hp ba cu trc c bn ny. 2.3.1. Cu trc bus Trong cu trc ny tt c cc thnh vin ca mng u c ni trc tip vi mt ng dn chung. c im c bn ca cu trc bus l vic s dng chung mt ng dn duy nht cho tt c cc trm, v th tit kim c cp dn v cng lp t. C th phn bit ba kiu cu hnh trong cu trc bus: daisy-chain, trunk-line/drop-line v mch vng khng tch cc nh hnh 2.8 di y:

Hai cu hnh u c xp vo kiu cu trc ng thng, bi hai u ng truyn khng khp kn. Vi daisy-chain, mi trm c ni mng trc tip ti giao l ca hai on dy dn, khng qua mt on dy ni ph no. Ngc li, trong cu trc trunk-line/drop-line, mi trm c ni qua mt ng nhnh (drop-line) n ng trc (trunk-line). Cn mch vng khng tch cc thc cht ch khc vi trunk-line/drop-line ch ng truyn c khp kn. Nhn xt:

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-Tit kim dy dn, n gin, d thc hin nn cu trc ny ph bin nht trong cc h thng mng truyn thng cng nghip. Trng hp mt trm khng lm vic (do hng hc, ct ngun,) th khng nh hng ti cc trm cn li. Mt s h thng cn tch mt trm ra khi mng hoc thay th mt trm trong khi c h thng vn hot ng bnh thng. - Vic dng chung mt ng dn i hi mt phng php phn chia thi gian s dng thch hp trnh xung t tn hiu gi l phng php truy nhp mi trng hay truy nhp bus. Nguyn tc truyn thng c thc hin nh sau: Ti mt thi im nht nh ch c mt thnh vin trong mng c gi tn hiu, cn cc thnh vin khc ch c quyn nhn. Ngoi vic cn kim sot truy nhp mi trng, cu trc bus c nhng nhc im sau: ++ Mt tn hiu gi i c th ti tt c cc trm v theo mt trnh t khng kim sot c. V vy phi thc hin phng php gn a ch (logic) theo kiu th cng cho tng trm. Trong thc t, cng vic gn a ch ny gy ra khng t kh khn. ++ Tt c cc trm u c kh nng pht v phi lun lun nghe ng dn pht hin ra mt thng tin c phi gi cho mnh hay khng, nn phi c thit k sao cho ti vi s trm ti a. y chnh l l do phi hn ch s trm trong mt on mng. Khi cn m rng mng phi dng thm cc b lp. ++ Chiu di dy dn thng tng i di, v vy i vi cu trc ng thng xy ra hin tng phn x ti mi u dy lm gim cht lng ca tn hiu. khc phc vn ny ngi ta chn hai du dy bng hai tr u cui. Vic s dng cc tr u cui cng lm tng ti ca h thng. ++ Trng hp ng dn b t hoc do ngn mch trong phn kt ni bus ca mt trm b hng u dn n ngng hot ng ca c h thng. Vic nh v li y cng gp rt nhiu kh khn. ++ Cu trc ng thng, lin kt a im gy kh khn trong vic p dng cc cng ngh truyn tn hiu mi nh s dng cp quang. Mt s v d mng cng nghip tiu biu c cu trc bus l PROFIBUS, CAN, WorldFIP, Foundation Fieldbus, LonWorks, AS-I v Ethernet. 2.3.2. Cu trc mch vng (tch cc) Cu trc mch vng c thit k sao cho cc thnh vin trong mng c ni t im ny n im kia mt cch tun t trong mt mch vng khp kn. Mi thnh vin u tham gia tch cc vo vic kim sot dng tn hiu. Khc vi cu trc ng thng, y tn hiu c truyn i theo mt chiu qui nh. Mi trm nhn c d liu t trm ng trc v chuyn tip sang trm ln cn ng sau. Qu trnh ny c lp li ti khi d liu quay tr v trm gi, n s c hy b. u im c bn ca mng cu trc theo kiu ny l mi nt ng thi c th l mt b khuch i, do vy khi thit k mng theo kiu cu trc vng c th thc hin vi khong cch v s trm rt ln. Mi trm c kh nng va nhn va pht tn hiu cng mt lc. Bi

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mi thnh vin ngn cch mch vng ra lm hai phn, v tn hiu ch c truyn theo mt chiu, nn bin php trnh xung t tn hiu thc hin n gin hn.

Trn hnh 2.9 c hai kiu mch vng c minh ha: ++ Vi kiu mch vng khng c iu khin trung tm, cc trm u bnh ng nh nhau trong quyn nhn v pht tn hiu. Nh vic kim sot ng dn s do cc trm t phn chia. ++ Vi kiu c iu khin trung tm, mt trm ch s m nhim vai tr kim sot vic truy nhp ng dn. Cu trc mch vng thc cht da trn c s lin kt im im, v vy thch hp cho vic s dng cc phng tin truyn tn hiu hin i nh cp quan, tia hng ngoi,Vic gn a ch cho cc thnh vin trong mng cng c th do mt trm ch thc hin mt cch hon ton t ng, cn c vo th t sp xp vt l ca cc trm trong mch vng. Mt u im tip theo ca cu trc mch vng l kh nng xc nh v tr xy ra s c v d t dy hay mt trm ngng lm vic. Trong trng hp c s c th mng ch c th tip tc hot ng vi mt ng dy d phng nh FDDI. M t cch gii quyt trong trng hp s c do ng dy v s c ti mt trm nh hnh 2.10 sau:

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Trong trng hp s c ng dy, cc trm ln cn vi im xy ra s c s t pht hin li ng dy v t ng chuyn mch sang ng dy ph, i vng qua v tr b li (by-pass). ng cong in nt m biu din mch kn sau khi dng bin php by-pass. Trong trng hp th hai, khi mt trm b hng, hai trm ln cn s t u tt, chuyn sang cu hnh ging nh daisy-chain. Mt k thut khc c p dng x l s c ti mt trm l dng cc b chuyn mch by-pass t ng, nh minh ha trn hnh 2.11. Mi trm thit b s c u vi mch vng nh b chuyn mch ny. Trong trng hp s c xy ra, b chuyn mch s t ng pht hin v ngn mch, b qua thit b c ni mng qua n.

Cu trc mch vng c s dng trong mt s h thng c tin cy cao nh INTERBUS, Token-Ring(IBM) v c bit l FDDI. 2.3.3. Cu trc hnh sao (hnh 2.12)

Cu trc hnh sao l mt cu trc mng c mt trm trung tm quan trng hn tt c cc nt khc, nt ny s iu khin hot ng truyn thng ca ton mng. Cc thnh vin khc c kt ni gin tip vi nhau qua trm trung tm. Tng t nh cu trc mch vng, c th nhn thy y kiu lin kt v mt vt l l im im. Tuy nhin, lin kt v mt logic vn c th l nhiu im. Nu trm trung tm ng vai tr tch cc, n c th m ng nhim v kim sot ton b vic truyn thng ca mng, cn nu khng s ch nh mt b chuyn mch. Nhn xt:

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Nhc im: ++ Trm trung tm gp s c s lm t lit ton b cc hot ng truyn thng trong mng. V vy, trm trung tm thng phi c tin cy rt cao. Ngi ta phn bit gia hai loi trung tm: Trm tch cc v trm th ng. Mt trm th ng ch c vai tr trung chuyn thng tin, trong khi mt trm tch cc kim sot ton b cc hot ng giao tip trong mng. ++ Tn dy dn, nu nh khong trung bnh gia cc trm nh hn khong cch t chng ti trm trung tm. i vi mng truyn thng cng nghip, cu trc hnh sao tm thy trong cc phm vi nh, v d cc b chia, thng dng vo mc ch m rng cc cu trc khc. Trong nhiu trng hp mt mng cu trc hnh sao v mt vt l li c cu trc logic nh mt h bus, bi cc trm vn c th t do lin lc nh khng c s tn ti ca trm trung tm. Chnh cc h thng mng Ethernet cng nghip ngy nay s dng ph bin cu trc ny kt hp vi k thut chuyn mch v phng php truyn dn tc cao. 2.3.4 Cu trc cy

Cu trc cy thc cht khng phi l mt cu trc c bn. Mt mng c cu trc cy chnh l s lin kt ca nhiu mng con c cu trc ng thng, mch vng hoc hnh sao nh hnh 2.13 minh ha. c trng ca cu trc cy l s phn cp ng dn. chia t ng trc ra cc ng nhnh, c th dng cc b ni tch cc (active coupler), hoc nu mun tng s trm cng nh phm vi ca mt mng ng nht c th dng cc b lp (repeater). Trong trng hp cc mng con ny hon ton khc loi th phi dng ti cc b lin kt mng khc nh bridge, router v gateway. Mt s h thng cho php xy dng cu trc cy cho mt mng ng nht l LonWorks, DeviceNet v AS-i. 2.4. Kin trc giao thc i vi mi h thng truyn thng, kin trc giao thc l c s cho vic tm hiu cc dch v cng nh hnh thc giao tip trong h thng. 2.4.1. Dch v truyn thng

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Mt h thng truyn thng cung cp dch v truyn thng cho cc thnh vin tham gia ni mng. Cc dch v c dng cho vic thc hin cc nhim v khc nhau nh trao i d liu, bo co trng thi, to lp cu hnh v tham s ha thit b trng, gim st thit b v ci t chng trnh. Cc dch v truyn thng do nh cung cp h thng truyn thng thc hin bng phn cng hoc phn mm. Vic khai thc cc dch v t pha ngi s dng phi thng qua phn mm giao din mng, to lp cc chng trnh phn mm ng dng, v d chng trnh iu khin, giao din ngi my (HMI) v iu khin gim st (SCADA). Cc giao din mng ny c th c ci t sn trn cc cng c phn mm chuyn dng (v d phn mm lp trnh PLC, phn mm SCADA, phn mm qun l mng) hoc qua cc th vin phn mm ph thng khc di dng cc hm dch v (nh C/C++, VisualBasic, Delphi,) Mi h thng truyn thng khc nhau c th qui nh mt chun ring v tp hp cc dch v truyn thng ca mnh. V d PROFIBUS nh ngha cc hm dch v khc so vi INTERBUS hay ControlNet. Mt phn mm chuyn dng khng nht thit phi h tr ton b cc dch v truyn thng ca mt h thng, nhng cng c th cng mt lc h tr nhiu h thng truyn thng khc nhau. V d vi mt cng c phn mm SCADA ta c th ng thi khai thc d liu t cc u o hay cc PLC lin kt vi cc bus trng khc nhau, nhng khng cn ti dch v h tr ci t chng trnh iu khin cho cc PLC. C th phn loi dch v truyn thng da theo cc cp khc nhau: ++ Cc dch v s cp (v d to v ngt ni) ++ Dch v cp thp (v d trao i d liu) ++ Dch v cao cp (to lp cu hnh, bo co trng thi) Mt dch v cp cao hn c th s dng cc dch v cp thp thc hin chc nng ca n. V d dch v to lp cu hnh hay bo co trng thi cui cng cng phi s dng dch v trao i d liu thc hin chc nng ca mn. Mt khc, trao i d liu thng i hi to v ngt ni. Phn cp dch v truyn thng cn c ngha l to s linh hot cho pha ngi s dng. Ty theo nhu cu v tin li hay hiu sut trao i thng tin m ngi ta c th quyt nh s dng mt dch v cp no. Vic thc hin tt c cc dch v c da trn cc nguyn hm dch v (service primitive) gm c: ++ yu cu (request) dch v, k hiu l req v d connect.req ++ Ch th (indication) nhn li phc v, k hiu l .ind v d connect.ind ++ p ng (response) dch v, k hiu l .res, v d connect.res ++ Xc nhn (confirmation) nhn c p ng, k hiu l .con, v d connect.con Da trn quan h gia bn cung cp dch v v bn yu cu dch v cng c th phn bit gia loi dch v c xc nhn v dch v khng xc nhn. Dch v c xc nhn i hi s dng c bn nguyn hm, trong khi dch v khng xc nhn b qua p ng v xc nhn.

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Biu tun t trn hnh 2.14 minh ha hai v d tiu biu cho hai trng hp ny l dch v to ni (connect) v ngt ni (disconnect).

2.4.2 Giao thc Bt c s giao tip no cng cn mt ngn ng chung cho cc i tc. Trong k thut truyn thng, bn cung cp dch v cng nh bn s dng dch v u phi tun th theo cc qui tc, th tc cho vic giao tip, gi l giao thc. Giao thc chnh l c s cho vic thc hin v s dng cc dch v truyn thng Mt qui chun giao thc bao gm cc thnh phn sau: ++ C php (syntax): Qui nh v cu trc bc in, gi d liu dng khi trao i, trong c phn thng tin hu ch (d liu) v cc thng tin b tr nh a ch, thng tin iu khin, thng tin kim li, ++ Ng ngha (semantic): Qui nh ngha c th ca tng phn trong mt bc in, nh phng php nh a ch, phng php bo ton d liu, th tc iu khin dng thng tin, x l li, ++ nh thi (timing): Qui nh v trnh t, th tc giao tip, ch truyn (ng b hay khng ng b), tc truyn thng, Vic thc hin mt dch v truyn thng trn c s cc giao thc tng ng c gi l x l giao thc. Ni mt cch khc, qu trnh x l giao thc c th l m ha (x l giao thc bn gi) v gii m (x l giao thc bn nhn). Tng t nh cc dch v truyn thng, c th phn bit cc giao thc cp thp v giao thc cao cp. Cc giao thc cao cp l c s cho cc dch v cao cp v cc giao thc cp thp l c s cho cc dch v cp thp. Giao thc cao cp gn vi ngi s dng, thng c thc hin bng phn mm. Mt s v d v giao thc cao cp l FTP (File Transfer Protocol) dng trong trao i file t xa, HTTP (Hypertext Transfer Protocol) dng trao i cc trang HTML trong cc ng dng Web, MMS (Manuafacturing Message Specification) dng trong t ng ha cng nghip. Giao thc cp thp gn vi phn cng, thng c thc hin trc tip bi cc mch in t. Mt s v d giao thc cp thp quen thuc l TCP/IP (Transmission Control

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Protocol/Internet Protocol) c dng ph bin trong Internet, HART (Highway Adresable Remote Transducer) dng trong iu khin qu trnh, HDLC (High Level Data-link Control) lm c s cho nhiu giao thc khc v UART dng trong a s cc giao din vt l ca cc h thng bus trng. Hai giao thc ni sau HDLC v UART c vai tr quan trng trong truyn thng cng nghip v v vy c gii thiu s lc di y. Giao thc HDLC: HDLC cho php ch truyn bit ni tip ng b hoc khng ng b. Mt bc in, hay cn gi l khung (frame) c cu trc nh sau:

Mi khung c khi u v kt thc bng mt c hiu (flag) vi dy bit 0111110. Dy bit ny c m bo khng bao gi xut hin trong cc phn thng tin khc qua phng php nhi bit (bit stuffing), tc c sau mt dy 5bit c gi tr 1 (11111) th mt bit 0 li c b sung vo. a ch tip theo cha a ch bn gi v bn nhn. Ty theo cch gn a ch 4 hoc 8 bit (tng ng vi 32 hoc 256 a ch khc nhau), ny c chiu di l 8 hoc 16bit. Trong HDLC c ba loi bc in, c phn bit qua thng tin iu khin (8bit), l: ++ Information Frames: Khung thng tin (I-Format) ++ Supervisory Frames: Khung gim st vn chuyn d liu (S-Format) ++ Unnumbered Frames: Khung b tr kim sot cc mi lin kt gia cc trm (U-Format). Cu trc ca thng tin iu khin c qui nh nh sau:

thng tin c di bin thin, cng c th trng nu nh bc in khng dng vo mc ch vn chuyn d liu. Sau thng tin l n dy bit kim li (FCS = Frame Check Sequence), dng vo mc ch bo ton d liu. Tc truyn thng tiu biu i vi HDLC t 9,6kbit/s n 2Mbit/s. + Giao thc UART: UART (Universal Asychronous Receiver/ Transmitter) l mt mch vi in t c s dng rt rng ri cho vic truyn bit ni tip cng nh chuyn i song song /ni tip gia ng truyn v bus my tnh. UART cho php la chn gia ch truyn mt chiu, hai chiu ng b hoc hai chiu khng ng b. Vic truyn ti c thc hin theo tng k t 7 hoc 8 bit, c b sung 2bit nh du u cui v mt bit kim tra li chn l (parity bit). V d vi k t 8bit c minh ha di y:

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Bit khi u (Start bit) bao gi cng l 0 v bit kt thc (Stop bit) bao gi cng l 1. Cc bit trong mt k t c truyn theo th t t bit thp (LSB) ti bit cao (MSB). Gi tr ca bit chn l P ph thuc vo cch chn: ++ Nu chn parity chn, th P =0 khi tng s bit 1 l chn ++ Nu chn parity l, th P=0 khi tng s bit 1 l l Nh tn ca n th hin, ch truyn khng ng b c s dng y, tc khng c mt tn hiu ring phc v cho vic ng b ha gia bn gi v bn nhn. Da vo cc bit u cui v tc truyn thng c t trc cho c hai bn, bn nhn thng tin phi t chnh nhp ly mu ca mnh ng b vi bn gi. 2.4.3. M hnh lp trao i d liu gia hai thit b, cc th tc, giao thc cn thit c th tng i phc tp. Thay v phi thc hin tt c cc bc cn thit trong mt module duy nht, c th chia nh thnh cc phn vic c th thc hin c lp. Trong m hnh lp, cc phn vic c sp xp theo chiu dc thnh tng lp, tng ng vi cc lp dch v v cc lp giao thc khc nhau. Mi lp gii quyt mt nhim v r rng phc v vic truyn thng. Mt dch v lp trn s dng dch v ca lp di ngay k n. thc hin mt dch v truyn thng, mi bc in c x l qua nhiu lp trn c s cc giao thc qui nh, gi l x l giao thc theo m hnh lp. Mi lp y c th thuc chc nng ca phn cng hoc phn mm. Cng lp cao hn th phn mm cng chim vai tr quan trng, trong khi vic x l giao thc cc lp di thng c cc vi mch in t trc tip thc hin. Hnh 2.15 di minh ha nguyn tc x l giao thc theo m hnh lp.

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ng t bn gi thng tin, qua mi lp t trn xung di, mt s thng tin b tr li c gn thm vo phn d liu do lp trn a xung, gi l u giao thc (protocol header). Bn cnh , thng tin cn truyn i c th c chia thnh nhiu bc in c nh s th t, hoc mt bc in c th tng hp nhiu ngun thng tin khc nhau. Ngi ta cng dng cc khi nim nh ng gi d liu hoc to khung ch cc thao tc ny. Mt qu trnh ngc li s din ra bn nhn thng tin. Cc phn header s c cc lp tng ng c, phn tch v tch ra trc khi gi tip ln lp trn. Cc bc in mang mt ngun thng tin s c tng hp li, hoc mt bc in mang nhiu ngun thng tin khc nhau s c phn chia tng ng. n lp trn cng, thng tin ngun c ti to. Vi m hnh phn lp, ngha ca giao thc mt ln na th hin r. ng nhin, thc hin truyn thng cn c hai i tc tham gia, vy phi tn ti cng mt tp hp cc hm phn lp ca trong hai thit b. Quan h giao tip y chnh l quan h gia cc lp tng ng s dng chung mt ngn ng, tc chung mt giao thc th mi c th trao i thng tin. Trong trng hp khc, cn c mt phn t trung gian hiu c hai giao thc, gi chung l b chuyn i, c th l bridge hay gateway ty theo lp giao thc ang quan tm. Vn mu cht y c th thc hin c vic chuyn i l s thng nht v dch v truyn thng ca cc lp tng ng trong hai h thng khc nhau. Nu hai h thng li qui nh cc chun khc nhau v dch v th vic chuyn i rt b hn ch v nhiu khi hon ton khng c ngha. V d, mt bn i hi ci t cc dch v cao cp nh ci t v kim sot chy chng trnh t xa, trong khi bn i tc ch cung cp dch v trao i d liu thun ty th vic chuyn i y khng c vai tr g cng nh khng th thc hin c. Tuy nhin, cng nhng dch v cp thp cng d c c hi a ra mt chun thng nht cho c hai pha.

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2.4.4. Kin trc giao thc OSI Theo m hnh OSI, chc nng hay dch v ca mt h thng truyn thng c chia thnh by lp, tng ng vi mi lp dch v l mt lp giao thc. Cc lp ny c th do phn cng hoc phn mm thc hin, tuy nhin chun ny khng cp ti chi tit mt i tc truyn thng phi thc hin tng lp nh th no. Mt lp trn thc t thc hin dch v ca mnh trn c s s dng cc dch v mt lp pha di v theo ng giao thc qui nh tng ng. Thng thng, cc dch v cp thp do phn cng (cc vi mch in t) thc hin, trong khi cc dch v cao cp do phn mm (h iu hnh, phn mm iu khin, phn mm ng dng) m nhim. Vic phn lp khng nhng c ngha quan trng trong vic m t, i chiu cc h thng truyn thng, m cn gip ch cho vic thit k cc thnh phn giao din mng. Mt lp bt k trong by lp c th thay i trong cch thc hin m khng nh hng ti cc lp khc, chng no n gi nguyn giao din vi lp trn v lp di n. V y l mt m hnh qui chiu c tnh cht dng tham kho, khng phi h thng truyn thng no cng thc hin y c by lp . V d, v l do hiu sut trao i thng tin v gi thnh thc hin, i vi cc h thng bus trng thng thng ch thc hin cc lp 1,2,7. Trong cc trng hp ny, c th mt s lp khng thc s cn thit hoc chc nng ca chng c ghp vi mt lp khc (v d lp ng dng). Mt m hnh qui chiu to ra c s, nhng khng m bo kh nng tng tc gia cc h thng truyn thng, cc thit b truyn thng khc nhau. Vi vic nh ngha by lp, OSI a ra mt m hnh tru tng cho cc qu trnh giao tip phn cp. Nu hai h thng thc hin cng cc dch v v trn c s mt giao thc ging nhau mt lp, th c ngha l hai h thng c kh nng tng tc lp . M hnh OSI c th coi nh mt cng trnh khung, h tr vic pht trin v c t cc chun giao thc. Cc lp trong m hnh qui chiu OSI v quan h gia chng vi nhau c minh ha trn hnh 2.16. Bn thn mi trng truyn thng v cc chng trnh ng dng khng thuc phm vi cp ca chun OSI.

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++ Lp ng dng: c chc nng cung cp cc dch v cao cp cho ngi s dng v cc chng trnh ng dng. V d, c th sp xp cc dch v v giao thc theo chun MMS cng nh cc dn xut ca n s dng trong mt s h thng bus trng thuc lp ng dng. Cc dch v thuc lp ng dng hu ht c thc hin bng phn mm. Thnh phn phn mm ny c th c nhng sn trong cc linh kin giao din mng hoc di dng phn mm iu khin (drivers) c th np ln khi cn thit, v/hoc mt th vin cho ngn ng lp trnh chuyn dng hoc ngn ng lp trnh ph thng. c kh nng s dng d dng trong mt chng trnh ng dng (v d iu khin c s hoc iu khin gim st), nhiu h thng cung cp cc dch v ny thng qua cc khi chc nng (function block). i vi cc thit b trng thng minh, cc khi chc nng ny khng ch n thun mang tnh cht ca dch v truyn thng, m cn tch hp c mt s chc nng x l thng tin, thm ch c iu khin ti ch. y cng chnh l xu hng mi trong vic chun ha lp ng dng cho cc h thng bus trng, hng ti kin trc iu khin phn tn trit . ++ Lp biu din d liu (presentation layer): Trong mt mng truyn thng, v d mng my tnh, cc trm my tnh c th c kin trc rt khc nhau, s dng cc h iu hnh khc

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nhau v v vy cch biu din d liu ca chng cng c th rt khc nhau. S khc nhau trong cch biu din d liu c th l di khc nhau cho mt kiu d liu hoc cch sp xp cc byte khc nhau trong mt kiu nhiu byte, hoc s dng bng m k t khc nhau. V d, mt s nguyn c kiu integer c th biu din bng 2byte, 4byte hoc 8byte, ty theo th h CPU, h iu hnh v m hnh lp trnh. Ngay c mt kiu integer c chiu di 2byte cng c hai cch sp xp th t byte gi tr cao ng trc hay ng sau byte gi tr thp. Mt v d khc l s khc nhau trong cch s dng bng m k t trong cc h thng vn chuyn th in t, gy ra khng t rc ri cho ngi s dng thuc cc nc khng ni ting Anh. Trong khi a s cc h thng mi s dng 8bit, th mt s h thng c ch x l c k t 7bit, v vy mt s k t c m ha vi gi tr ln hn 127 b hiu sai. Chc nng ca lp biu din d liu l chuyn i cc dng biu din d liu khc nhau v c php thnh mt dng chun, nhm to iu kin cho cc i tc truyn thng c th hiu c nhau mc d chng s dng cc kiu d liu khc nhau. Ni mt cch khc, lp biu din d liu gii phng s ph thuc ca lp ng dng vo cc phng php biu din d liu khc khau. Ngoi ra, lp ny cn c th cung cp mt s dch v bo mt d liu, v d qua phng php s dng m kha. Nu nh cch biu din d liu c thng nht, chun ha th chc nng ny khng nht thit phi tc ring thnh mt lp c lp, m c th kt hp thc hin trn lp ng dng n gin ha v nng cao hiu sut ca vic x l giao thc. y chnh l mt c trng trong cc h thng bus trng. ++ Lp kim sot ni/lp phin (session layer) Mt qu trnh truyn thng, v d trao i d liu gia hai chng trnh ng dng thuc hai nt mng, thng c tin hnh thnh nhiu giai on. Lp kim sot ni c chc nng kim sot mi lin kt truyn thng gia cc chng trnh ng dng, bao gm cc vic to lp, qun l v kt thc cc ng ni gia cc ng dng i tc. Cn phi nhc li rng, mi lin kt gia cc chng trnh ng dng mang tnh cht logic; thng qua mt mi lin kt vt l (gia hai trm, gia hai nt mng) c th tn ti song song nhiu ng ni logic. Thng thng kim sot ni thuc chc nng ca h iu hnh. thc hin cc ng ni gia hai ng dng i tc, h iu hnh c th to cc qu trnh tnh ton song song (cnh tranh). Nh vy, nhim v ng b ha cc qu trnh tnh ton ny i vi vic s dng chung mt giao din mng cng thuc chc nng ca lp kim sot ni. Chnh v th, lp ny cn c tn l lp ng b ha. Trong h thng bus trng, quan h ni gia cc chng trnh ng dng c xc nh sn (quan h tnh) nn lp kim sot ni khng ng vai tr g ng k. i vi mt s h thng khc, chc nng ca lp ny c y ln kt hp vi lp ng dng v l do hiu sut x l truyn thng. ++ Lp vn chuyn (transport layer)

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Bt k bn cht ca cc ng dng cn trao i d liu, iu cn thit l d liu phi c trao i mt cch tin cy. Khi mt khi d liu c chuyn i thnh tng gi, cn phi m bo rng tt c cc gi u n ch v theo ng trnh t chng c chuyn i. Chc nng ca lp vn chuyn l cung cp cc dch v cho vic thc hin vn chuyn d liu gia cc chng trnh ng dng mt cch tin cy, bao gm c trch nhim khc phc li v iu khin lu thng. Nh vy m cc lp trn c th thc hin c cc chc nng cao cp m khng cn quan tm ti c ch vn chuyn d liu c th. Cc nhim v c th ca lp vn chuyn bao gm: ++ Qun l v tn hnh thc cho cc trm s dng ++ nh v cc i tc truyn thng qua tn hnh thc v/hoc a ch ++X l li v kim sot dng thng tin, trong c c vic lp li quan h lin kt v thc hin cc th tc gi li d liu khi cn thit. ++ Dn knh cc ngun d liu khc nhau ++ ng b ha gia cc trm i tc thc hin vn chuyn mt cch hiu qu, tin cy, mt d liu cn chuyn i c th c chia thnh nhiu n v vn chuyn (data segment unit) c nh s th t kim sot trc khi b sung cc thng tin kim sot lu thng. Do cc c im ring ca mng truyn thng cng nghip, mt s nhim v c th ca lp vn chuyn tr nn khng cn thit, v d vic dn knh hoc kim sot lu thng. Mt s chc nng cn li c dn ln kt hp vi lp ng dng tin vic thc hin v to iu kin cho ngi s dng t chn phng n ti u ha v nng cao hiu sut truyn thng. ++ Lp mng (network layer) Mt h thng mng din rng (v d Internet) l s lin kt ca nhiu mng tn ti c lp. Mi mng ny u c mt khng gian a ch v c mt cch nh a ch ring bit, s dng cng ngh truyn thng khc nhau. Mt bc in i t i tc A sang mt i tc B mt mng khc c th qua nhiu ng khc nhau, thi gian, qung ng vn chuyn v cht lng ng truyn v th cng khc nhau. Lp mng c trch nhim tm ng i ti u (routing) cho vic vn chuyn d liu, gii phng s ph thuc ca cc lp bn trn vo phng thc chuyn giao d liu v cng ngh chuyn mch dng kt ni cc h thng khc nhau. Tiu chun ti u y hon ton da trn yu cu ca cc i tc, v d yu cu thi gian, qung ng, v gi thnh dch v hay yu cu v cht lng dch v. Vic xy dng v hy b cc quan h lin kt gia cc nt mng cng thuc trch nhim ca lp mng. C th nhn thy, lp mng khng c ngha i vi mt h thng truyn thng cng nghip, bi y hoc khng c nhu cu trao i d liu gia hai trm thuc hai mng khc nhau, hoc vic trao i c thc hin gin tip thng qua chng trnh ng dng (khng thuc lp no trong m hnh OSI). Vic thc hin trao i d liu thng qua chng trnh

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ng dng xut pht t l do l ngi s dng (lp trnh) mun c s kim sot trc tip ti ng i ca mt bc in m bo tnh nng thi gian thc, ch khng mun ph thuc vo thut ton tm ng i ti u ca cc b router. Cng v vy, cc b router thng dng trong lin kt mng hon ton khng c vai tr g trong cc h thng bus trng. ++ Lp lin kt d liu (data link layer): Lp lin kt d liu c trch nhim truyn dn d liu mt cch tin cy thng qua mi lin kt vt l, trong bao gm vic iu khin truy nhp mi trng truyn dn v bo ton d liu. Lp lin kt d liu cng thng c chia thnh hai lp con tng ng vi hai chc nng l: Lp iu khin truy nhp mi trng (medium access control-MAC) v lp iu khin lin kt logic (logical link control - LLC). Trong mt s h thng, lp lin kt d liu c th m nhim thm cc chc nng khc nh kim sot lu thng v ng b ha vic chuyn giao cc khung d liu. thc hin chc nng bo ton d liu, thng tin nhn c t lp pha trn c ng gi thnh cc bc in c chiu di hp l (frame). Cc khung d liu ny cha cc thng tin b sung phc v mc ch kim li, kim sot lu thng v ng b ha. Lp lin kt d liu bn pha nhn thng tin s da vo cc thng tin ny xc nh tnh chnh xc ca d liu, sp xp cc khung li theo ng trnh t v khi phc li thng tin chuyn tip ln lp trn n. ++ Lp vt l (physical layer): Lp vt l l lp di cng trong m hnh phn lp chc nng truyn thng ca mt trm thit b. Lp ny m nhim ton b cng vic truyn dn d liu bng phng tin vt l. Cc qui nh y m t giao din vt l gia mt trm thit b v mi trng truyn thng: Cc chi tit v cu trc mng (bus, cy, hnh sao,) K thut truyn dn (RS-485, MBP, truyn cp quang,) Phng php m ha bit (NRZ, Manchester, FSK,) Ch truyn ti (di rng/di c s/ di mang, ng b/ khng ng b) Cc tc truyn cho php Giao din c hc (phch cm, gic cm,) Cn lu rng lp vt l hon ton khng cp ti mi trng truyn thng, m ch ni ti giao din vi n. C th ni, qui nh v mi trng truyn thng nm ngoi phm vi ca m hnh OSI. Lp vt l cn c chun ha sao cho mt h thng truyn thng c s la chn gia mt vi kh nng khc nhau. Trong cc h thng bus trng, s la chn ny khng ln qu, hu ht da trn mt vi chun v k thut c bn. Tin trnh thc hin giao tip theo m hnh OSI c minh ha bng mt my tnh iu khin v mt thit b o thng minh, nh th hin trn hnh 2.17:

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Cc mi tn nt gch chm biu th quan h giao tip logic gia cc lp tng ng thuc hai trm. Lp vt l thuc trm A c ni trc tip vi lp vt l thuc trm B qua cp truyn. Trong thc t, cc chc nng thuc lp vt l v lp lin kt d liu c thc hin hu ht trn cc mch vi in t ca phn giao din mng. i vi my tnh iu khin hoc thit b o th phn giao din mng c th tch hp trong phn x l trung tm, hoc di dng mt module ring. Khi chng trnh iu khin trm A cn cp nht gi tr o, n s s dng dch v trao i d liu lp ng dng gi mt yu cu ti trm B. Trong thc t, qu trnh ny c th c thc hin n gin bng cch gi mt hm trong th vin giao tip ca mng c s dng. Quan h ni gia hai trm c thit lp sn. Lp ng dng bn A x l yu cu ca chng trnh iu khin v chuyn tip m lnh xung lp pha di lp biu din d liu. Lp ny biu din m lnh thnh mt dy bit c di v th t qui c, sau chuyn tip xung lp kim sot ni. Lp kim sot ni s b sung thng tin phn bit yu cu cp nht d liu xut pht t quan h ni logic no, t qu trnh tnh ton no. Bc ny tr nn cn thit khi trong mt chng trnh ng dng c nhiu qu trnh tnh ton cnh tranh (task) cn phi s dng dch v trao i d liu, v kt qu cp nht d liu phi c a tr v ng ni yu cu. Khi d liu giao thc (PDU) t lp kim sot ni chuyn xung c lp vn chuyn sp xp mt knh truyn ti v m bo yu cu s c chuyn ti bn B mt cch tin cy. S dng dch v chuyn mch v tm ng i ti u ca lp mng, mt s thng tin s c b sung vo bc in cn truyn nu cn thit. Tip theo, lp lin kt d liu gn thm cc thng tin bo ton d liu, s dng th tc truy nhp mi trng chuyn bc in xung lp vt l. Cui cng, cc vi mch in t di lp vt l (v d cc b thu pht RS-485) chuyn ha dy bit sang mt dng tn hiu thch hp vi ng truyn (m ha bit) gi sang bn B, vi mt tc truyn hay tc m ha bit theo qui c.

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Qu trnh ngc li din ra bn B. Qua lp vt l, tn hiu nhn c c gii m v dy bit d liu c khi phc. Mi lp pha trn s phn tch phn thng tin b sung ca mnh thc hin cc chc nng tng ng. Trc khi chuyn ln lp trn tip theo, phn thng tin ny c tch ra. ng nhin, cc qu trnh ny i hi hai lp i tc ca hai bn phi hiu c thng tin c cu trc v ngha nh th no, tc l phi s dng cng mt giao thc. Cui cng, chng trnh thu nhn d liu bn thit b o nhn c yu cu v chuyn gi tr o cp nht tr li trm A cng theo ng trnh t nh trn.\ 2.4.5. Kin trc giao thc TCP/IP (Transmission Control Protocol/Internet Protocol) Kin trc giao thc TCP/IP v i chiu vi m hnh OSI c minh ha trn hnh 2.18

++ Lp ng dng: Lp ng dng thc hin cc chc nng h tr cn thit cho nhiu ng dng khc nhau. Vi mi loi ng dng cn mt module ring bit, v d FTP (File Transfer Protocol) cho chuyn giao file, TELNET cho lm vic vi trm ch t xa, SMTP (Simple Mail Transfer Protocol) cho chuyn th in t, SNMP (Simple Network Management Protocol) cho qun tr mng v DNS (Domain Name Service) phc v qun l v tra cu danh sch tn v a ch Internet. ++ Lp vn chuyn: C ch m bo d liu c vn chuyn mt cch tin cy hon ton khng ph thuc vo c tnh ca cc ng dng s dng d liu. Chnh v th, c ch ny c sp xp vo mt lp c lp tt c cc ng dng khc nhau c th s dng chung, c gi l lp vn chuyn. C th ni, TCP l giao thc tiu biu nht, ph bin nht phc v vic thc hin chc nng ni trn. TCP h tr vic trao i d liu trn c s dch v c ni.

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Bn cnh TCP, giao thc UDP (User Data Protocol) cng c s dng cho lp vn chuyn. Khc vi TCP, UDP cung cp dch v khng c ni kt cho vic gi d liu, hn na cng khng bo m tuyt i n ch, trnh t n ca cc gi d liu. Nhng UDP li n gin v hiu sut, ch i hi mt c ch x l giao thc ti thiu, v vy thng c dng lm c s thc hin cc giao thc cao cp theo yu cu ring ca ngi s dng (v d giao thc SNMP). ++ Lp Internet: c chc nng chuyn giao d liu gia nhiu mng c lin kt vi nhau. Giao thc IP c s dng chnh lp ny, c thc hin khng nhng cc thit b u cui, cc b router. Mt router chnh l mt thit b x l giao thc dng lin kt hai mng, c chc nng chuyn giao d liu t mt mng ny sang mt mng khc, trong c c nhim v tm ng i ti u. ++ Lp truy nhp mng: Lp truy nhp mng lin quan ti vic trao i d liu gia hai trm thit b trong cng mt mng. Cc chc nng bao gm vic kim sot truy nhp mi trng truyn dn, kim li v lu thng d liu, ging nh lp lin kt d liu trong m hnh OSI. ++ Lp vt l: cp ti giao din vt l gia mt thit b truyn d liu (v d my PC, PLC) vi mi trng truyn dn hay mng, trong c c tnh tn hiu, ch truyn, tc truyn v cu trc c hc cc phch cm/ gic cm. 2.4.6 Chun MMS (Manufacturing Message Specification) MMS qui nh mt tp hp cc dch v chun cho vic trao i d liu thi gian thc cng nh thng tin iu khin gim st. Cc dch v ny cng nh cc giao thc tng ng c chun ha trong ISO/IEC 9506. MMS c xut pht t MAP (Manufacturing Automation Protocol). Mc d MAP khng tr thnh giao thc truyn thng thng nht cho cng nghip t ng ha, n tc ng c tnh cht nh hng ti cc pht trin sau ny. Trn c s ca MAP, cc dch v truyn thng c nh ngha trong MMS to c s quan trng trong vic xy dng lp ng dng trong nhiu h thng bus trng. Mt s v d nh FMS (Fieldbus Message Specification) ca PROFIBUS, PMS (Peripheral Message Specification) ca INTERBUS, MPS (Manufacturing Periodic/aperiodic Services) ca WorldFIP v RAC (Remote Access Control) ca Bitbus u l cc tp con cc dch v MMS. Cc dch v c nh ngha trong MMS c tnh cht thng dng v a dng, c th thch hp vi rt nhiu loi thit b, nhiu ng dng v ngnh cng nghip khc nhau. V d, dch v Read cho php mt chng trnh ng dng hoc mt thit b c mt hoc nhiu bin mt lc t mt chng trnh ng dng hoc mt thit b khc. Bt k mt thit b l PLC hay robot, mt chng trnh iu khin t ng hay chng trnh iu khin gim st, cc dch v v thng bo MMS u nh nhau. V c bn, MMS nh ngha:

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++ Hn 80 dch v truyn thng thng dng cho cc h thng bus, trong c kim sot ng ni, truy nhp bin, iu khin s kin, ci t v can thip chng trnh. ++ Mt giao thc qui nh cu trc d liu cho vic chuyn giao tham s ca cc dch v ++ M hnh mt s i tng o, i din cho cc i tng vt l (my mc, robot,) ++ Mt c ch Client/Server trong quan h gia cc i tc truyn thng. Chun ISO/IEC 9506 bao gm hai phn ct li sau: ++ Phn 1: c t dch v, nh ngha m hnh thit b sn xut o VMD (Virtual Manufacturing Device), cc dch v trao i gia cc nt mng, cc thuc tnh cng nh tham s tng ng vi VMD v cc dch v. ++ Phn 2: c t giao thc, nh ngha trnh t cc thng bo c gi i trong mng, cu trc v kiu m ha thng bo, tng tc gia MMS vi cc lp khc trong m hnh OSI, MMS s dng chun lp biu din d liu ASN.1 (Abstract Notation Number One ISO 8824) c t cu trc cc thng bo. Ngoi ra, bn phn ph tip theo c gi l cc chun i km (Companion Standard) m rng phn ct li nhm thch ng cho cc lnh vc ng dng iu khin khc nhau nh Robot Control, Numeric Control, Programmable Controller v Process Control. Mt trong nhng c trng ca MMS l m hnh i tng VMD. Trn quan im hng i tng, m hnh VMD cho php cc thit b ng vai tr mt server, cung cp cc dch v cho cc client thng qua cc i tng o. Cc i tng o ny i din cho nhng i tng khc nhau trong mt h thng k thut, trong c c cc bin, chng trnh, s kin,Mi chng trnh ng dng c th ng thi ng vai tr server v client. M hnh VMD nh ngha cc i tng sau: ++ VMD: Bn thn VMD c coi l mt i tng hp thnh t cc i tng khc, i din cho ton b mt thit b. ++ Domain: i din mt phn nh c lin kt trong mt VMD, v d phn nh cho mt chng trnh c th np xung (download) v np ln (upload) c. ++ Program Invocation: Mt chng trnh chy trong b nh chnh c hp thnh bi mt hoc nhiu domain. ++ Variable: Mt bin d liu c kiu (v d s nguyn, s thc du phy ng, mng) ++ Kiu: M t cu trc v ngha ca d liu cha trong mt bin ++ Named Variable List: Mt danh sch nhiu bin c tn. ++ Semaphore: Mt i tng dng kim sot vic truy nhp cnh tranh mt ti nguyn chung (v d b nh, CPU, cng vo/ra) ++ Operator Station: Mt trm c mn hnh v bn phm dng cho thao tc vin vn hnh qu trnh. ++ Event Condition: Mt i tng i din cho trng thi ca mt s kin

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++ Event Action: Mt i tng i din hnh ng c thc hin khi trng thi ca mt s kin thay i. ++ Event Enrollment: Mt i tng i din cho mt chng trnh ng dng mng c thng bo khi trng thi ca mt s kin thay i. ++ Journal: Mt i tng ghi li din bin ca cc s kin v bin theo thi gian. ++ File: Mt file trong mt trm server ++ Transaction: i din mt yu cu dch v MMS ring bit Cc loi dch v tng ng vi cc i tng c bn c gii thiu tm tt trong bng 2.1 di y:

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2.5. Truy nhp bus 2.5.1. t vn - Trong cc h thng mng truyn thng cng nghip th cc h thng c cu trc dng bus, hay cc h thng bus ng vai tr quan trng nht v nhng l do sau: + Chi ph t cho dy dn + D thc hin lp t + Linh hot + Thch hp cho vic truyn dn trong phm vi khong cch va v nh

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- Trong mt mng c cu trc bus, cc thnh vin phi chia nhau thi gian s dng ng dn. trnh s xung t v tn hiu gy ra sai lch v thng tin, mi thi im trn mt ng dn ch duy nht mt in tn c php truyn i. Chnh v vy mng phi c iu khin sao cho ti mt thi im nht nh th ch mt thnh vin trong mng mun nhn thng tin th khng hn ch. Mt trong nhng vn quan trng hng u nh hng ti cht lng ca mi h thng bus l phng php phn chia thi gian gi thng tin trn ng dn hay phng php truy nhn bus. - Lu rng, mt s cu trc khc khng phi dng bus, vn xung t tn hiu cng c th xy ra, tuy khng hiu nhin nh cu trc bus. V d i vi cu trc mch vng, mi trm khng phi bao gi cng c kh nng khng ch hon ton tn hiu i qua n. Hay cu trc hnh sao, c th trm trung tm khng c vai tr ch ng, m ch l b chia tn hiu nn kh nng gy xung t khng th trnh khi. Trong cc cu trc ny ta vn cn mt bit php phn chia quyn truy nhp, tuy c th n gin hn so vi cu trc bus. Chnh v th, khi nim truy nhp mi trng cng c dng thay cho truy nhp bus. Tuy nhin, ging nh cch dng khi nim chung bus trng khng ch dng li cc h thng c cu trc bus, truy nhp bus cng thng c dng nh mt khi nim chung. - Phng php truy nhp bus l mt trong nhng vn c bn, i vi cc h thng bus, bi mi phng php c nhng nh hng khc nhau ti cc tnh nng k thut ca h thng. C th, ta phi quan tm ti t nht 3 kha cnh: tin cy, tnh nng thi gian thc v hiu sut s dng ng truyn. Tnh nng thi gian thc y l kh nng p ng nhu cu trao i thng tin mt cch kp thi v tin cy. Cn hiu sut s dng ng truyn l mc khai thc, s dng ng truyn. - Ba yu t lin quan ti vic nh gi tnh nng thi gian thc l thi gian p ng ti a, chu k bus v rung. Thi gian p ng ti a i vi mi trm l thi gian ti a m h thng truyn thng cn p ng mt nhu cu trao i d liu ca trm vi mt trm bt k khc. R rng, thi gian p ng ti a khng phi l mt thng s c nh, m l mt hm ca di d liu cn trao i. Tuy vy, trong mt ng dng c th ta thng bit trc di d liu ti a cng nh di d liu tiu biu - Do c trng trong k thut t ng ha, a s cc h thng bus c s dng lnh vc ny lm vic theo chu k. Ch mt s cc hot ng truyn thng xy ra bt thng (v d thng tin cnh bo, d liu tham s,...), cn phn ln cc d liu c trao i nh k theo chu k tun hon ca bus. Chu k bus l khong thi gian ti thiu m sau cc hot ng truyn thng chnh lp li nh c. Trong iu khin t ng, chu k bus nh hng ti s chnh xc ca chu k lu mu tn hiu. Lu s khc nhau gia chu k bus v nhp bus. - Thi gian p ng v chu k bus c lin quan vi nhau, nhng khng mc rng buc. Chu k bus ln thng s lm tng thi gian p ng. Tuy nhin, thi gian p ng ti a c th nh hn hoc ln hn mt chu k bus, ph thuc vo phng php truy nhp bus.

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- Trong mt s h bus, v d Foundation Fieldbus Hl, khi nim chu k bus khng c ngha bi cc hot ng giao tip tun hon theo chu k c phn tn v thc hin theo mt lch trnh thi gian. Khi , rung l mt thng s quan trng nh gi tnh nng thi gian thc. rung c hiu khong thi gian sai lch gia thi im thc t mt trm gi c d liu so vi thi im lp lch, cng nh gia thi im thc t mt trm nhn c d liu so vi thi im mong mun trong lch trnh. - Hiu sut s dng ng truyn c tnh bng phn trm thi gian ng truyn c s dng thc s hiu qu vo vic truyn ti d liu. i lng ny ph thuc vo mt lu thng v vo phng php truy nhp bus. Mt lu thng thp dn n hiu sut thp, nhng ngc li mt lu thng qu cao cng dn n vn n tc lu thng v lm gim hiu sut. t c hiu sut s dng ng truyn ti a, cn phi tnh ton hoc th nghim tm ra cc thng s cho thit k cu hnh mng, trn c s phng php truy nhp bus c p dng. - C th phn loi cch truy nhp bus thnh nhm cc phng php tin nh v nhm cc phng php ngu nhin (hnh 2.19). Vi cc phng php tin nh, trnh t truy nhp bus c xc nh r rng. Vic truy nhp bus c kim sot cht ch theo cch tp trung mt trm ch (phng php Master/Slave), theo s qui nh trc v thi gian (phng php TDMA) hoc phn tn bi cc thnh vin (phng php Token Passing). Nu mi hot ng truyn thng c hn ch bi mt khong thi gian hoc mt di d liu nht nh, th thi gian p ng ti a cng nh chu k bus c th tnh ton c. Cc h thng ny v th c gi c tnh nng thi gian thc.

Hnh 2.19: Phn loi cc phng php truy nhp bus Ngc li, trong cc phng php ngu nhin trnh t truy nhp bus khng c quy nh cht ch trc, m xy ra hon ton theo nhu cu ca cc trm. Mi thnh vin trong mng c th th truy nhp bus gi thng tin i bt c lc no. loi tr tc hi ca vic xung t gy nn, c nhng phng php ph bin nh nhn b