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Serial Communication

EIA RS422 and RS485

Recommended StandardThe purpose of this course note is to attempt to describe the main elements in SerialCommunication. This application note attempts to cover enough technical details ofRS422 and RS485.

EIA- Electroonic Industries Association

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Introduction

The purpose of this note is to describe the

main elements of an RS-422 and RS-485

system.

This note attempts to cover enough technical

details so that the system designer will have

considered all the important aspects in the

data system design.

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Balanced Line Drivers

In a balanced differentialsystem the voltage producedby the driver appears across apair of signal lines thattransmit only one signal. (2-6V)

C: Signal Ground. It isnt usedby a balanced line receiver indetermining the logic state ofthe data line.

ENABLE: (Tristate condition ofthe driver)

RS-485 diver must haveENABLE control signal.

RS-422 driver may have thissignal, but it is not alwaysrequired.

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Schmitt Trigger using Op-Amp

vout=A(v2v1)

The comparator will switch when V+=0. Then

The effect of using a Schmitt trigger

(B) instead of a comparator (A).

Typical hysteresis curve (Non-inverting)

Non-inverting Schmitt trigger

Inverting Schmitt trigger

The difference between the trip points

is the hysteresis H and is given as

sss V

R

RVR

RVR

RH

2

1

2

1

2

1 2)(

Let's say you detect a low-to-high transition at 2.5 V. A

100 mV hysteresis would mean that the low-to-high

transition is detected at 2.55 V and the high-to-low

transition is detected at 2.45 V, a 100 mV difference.

Vcc = Voltage Common Collector Bipolar

Vee = Voltage Common Emitter BipolarVss = Voltage Common Source FET

Vdd = Voltage Common Drain FET

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EIA Standard RS-422 Data Transmission Each generator can drive up

to 10 receivers.

The 2 signaling states of the lineare define as follows:

When the A terminal of thedriver is negative with respectto the B terminal, the line is

in a binary 1 (MARK or OFF)state.

When the A terminal of the

driver is positive with respectto the B terminal, the line is

in a binary 0 (SPACE or ON)

state. Like RS232, the data bits are

transmitted in reverse order.

The terminator is placed at the end of a transmission

line or daisy chain bus, designed to match impedance and

hence minimize signal reflections.

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Figure shows the condition of the

voltage of the balanced line for

an RS-232 to RS-422 converter

when the line is in the idle

condition or OFF state.

The A terminal is equivalent to

the - designation.

The B terminal is equivalent to

the + designation.

The same relationship alsoapplies for RS-485 systems.

RS-422 can withstand a common

mode voltage (Vcm) of 7 volts

EIA Standard RS-422 Data Transmission

DVM: Digital voltmeter

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As many as 32 driver/receiver pairs can share a multi drop network

Many characteristics of the drivers and the receiver are the same as RS-

422.

The range of the common mode voltage Vcm that the driver and receiver

can tolerate is expanded to +12 to -7 volts. The driver can be disconnected or tristated from the line.

The signal ground line is also recommended in an RS-485 system

EIA Standard RS-485 Data Transmission

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Tristate control of an RS-485 device

using RTS

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Send data control of an RS-485 device

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Biasing an RS-485 Network

In order to maintain the proper idle voltage state (all drivers are tristated),

bias resistors are must be applied to force the data lines to the idle

condition.

Example: 10 node, RS-485 network with two 120

termination resistors

T i t t ti f

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Transient protection of

RS-422 and RS-485 system

Isolated RS-485 device

RS-485 Device with signal ground

connected to chassis ground

Signal ground connection between two nodes

with 100 resistor

Isolated node with shunt protection to earth ground

Isolated port with ungrounded shunt protection

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Surge Transient protection of

RS-422 and RS-485 system

Fused port protection

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SN65176B, SN75176B

DIFFERENTIAL BUS TRANSCEIVERS

The SN65176B and SN75176B differential bustransceivers are integrated circuits designed for

bidirectional data communication on multipoint

bus transmission lines.

They are designed for balanced transmission

lines and meet ANSI Standards TIA/EIA-422-B

and TIA/EIA-485-A The receiver features a minimum input

impedance of 12 k, an input sensitivity of 200

mV, and a typical input hysteresis of 50 mV.

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Full duplex

SN75179B DIFFERENTIAL DRIVER AND

RECEIVER PAIR

The SN75179B is a differential driver and

receiver pair designed for balancedtransmission-line applications and meets

TIA/EIA-422-B, TIA/EIA-485-A. It is designed to

improve the performance of full-duplex data

communications over long bus lines.

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Half duplex

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CONTROLLING THE DRIVER ENABLE

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CONVERT RS232 TO RS485

Half duplex

A MAX233 converts the RS-232 signals to TTL levels, and

the TTL signals connect to an SN75176Bthat provides the

RS-485 interface.

The MAX220MAX249 family of line drivers/receivers is

intended for all EIA/TIA-232E and V.28/V.24 communications

interfaces, particularly applications where 12V is not

available.

MAX232

MAX233

V.24 is a specification for single-ended communications that includes the definition of

connector pin allocations. It is used together with V.28 to define a specification for

serial asynchronous or synchronous communications.

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CONVERT RS232 TO RS485 Half duplex

Popular chip MAX232 is used to convert RS232 signals from/to TTL level. MAX485 converts TTL level signals to RS485

standard. RTSline is used to change transmission mode of MAX485. RTS signal is first converted to TTL level by MAX232 thenconnected to DEand RE/pins. 2 LEDs are used to indicate current state of the interface.

LM78LXX Series

Output voltages of 5.0V, 6.2V, 8.2V, 9.0V, 12V, 15V

The MAX481, MAX483, MAX485, MAX487MAX491, and MAX1487

are low-power transceivers for RS-485 and RS-422 communication.

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CONVERT RS232 TO RS485 Half duplex

This is a small RS232 to RS485 converter project it use for convert RS232 signal level to RS485 level multidrop. It can use with 32 slave to communicate withPC and embedded systems board for long distance less than 1.2Km( 4000 feet).

Specification

- 9-12Vdc power supply

- Small size only 2.8" X 1.45 "

- Use for RS485 multidrop 2 wire connection.

- Direct connect with PC on DB9 connecter.

- Use RTS signal to control direction .

- 32 Slaves (up to 256 slaves with some transceiver i.e. MAX3088)

J1 and J2 use for jump R terminate(120) at the end of communication line or last slaver. The 75176 use for transceiver.RTS active (logic 0 or -3 to -15V) when we need to send data .

Diode 1N4001 provides

protection against reverse

polarity connection of the

supplyAdding a resistor to a line may limit

damaging current flows that would

otherwise result from short high-voltage

transients, such as those caused by

electrostatic discharge (ESD).

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Data Format and Protocols

Information content passing through peer-to-peerconnection is packed in a very simple structure:

and are bothconfigurable via software (device configurationparameters)

Most common generic Handshakeare

available/selectable with RS232 interface: Hardware (RTS-CTS)

Software XON/XOFF

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