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AN-960

更新时间： 2024-01-04 20:03:49

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亚德诺 - ADI		/
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12页	252K
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RS-485/RS-422 CIRCUIT IMPLEMENTATION GUIDE

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AN-960 数据手册

AN-960

APPLICATION NOTE

One Technology Way • P. O. Box 9106 • Norwood, MA 02062-9106, U.S.A. • Tel: 781.329.4700 • Fax: 781.461.3113 • www.analog.com

RS-485/RS-422 Circuit Implementation Guide

by Hein Marais

INTRODUCTION

WHY USE DIFFERENTIAL DATA TRANSMISSION?

Industrial and instrumentation applications (I&I) require

transmission of data between multiple systems often over

very long distances. The RS-485 bus standard is one of the

most widely used physical layer bus designs in I&I applica-

tions. The key features of RS-485 that make it ideal for use

in I&I communications applications are

The main reason why RS-485 can communicate over long

distances is the use of differential or balanced lines. A com-

munication channel requires a dedicated pair of signal lines

to exchange information. The voltage on one line equals the

inverse of the voltage on the other line.

TIA/EIA-485-A designates the two lines in this differential pair

as A and B. Line A is more positive than Line B (V_OA> V_OB) on

the driver output if a logic high is received on the input of the

transmitter (DI = 1). If a logic low is received on the input of the

transmitter (DI = 0), the transmitter causes Line B to be more

positive than Line A (V_OB> V_OA). See Figure 1.

•

Long distance links—up to 4000 feet.

Bidirectional communications possible over a single pair of

twisted cables.

•

Differential transmission increases noise immunity and

decreases noise emissions.

Multiple drivers and receivers can be connected on the

same bus.

Wide common-mode range allows for differences in

ground potential between the driver and receiver.

Figure 1. Differential Transmitter and Receiver

TIA/EIA-485-A allow for data rates of up to 10 Mbps.

Devices meeting the TIA/EIA-485-A specifications do not

have to operate over the entire range and are not limited

to 10 Mbps.

If Line A is more positive than line B (V_IA− V_IB> 200 mV)

on the input of the receiver, the receiver output is a logic high

(RO = 1). If Line B is more positive than Line A (V_IB− V_IA

200 mV) on the input of the receiver, the receiver output is a

logic low (RO = 0).

The purpose of this application note is to discuss the imple-

mentation of RS-485/RS-422 in an industrial environment.

Applications for RS-485/RS-422 include process control

networks; industrial automation; remote terminals; building

automation, such as heating, ventilation, air conditioning

(HVAC), security systems; motor control; and motion control.

Figure 1 shows that a differential signaling interface circuit

consists of a driver with differential outputs and a receiver with

differential inputs. This circuit has increased noise performance

because the noise coupling into the system is equal on both

signals. One signal emits the opposite of the other signal and

electromagnetic fields cancel each other. This reduces the

electromagnetic interference (EMI) of the system.

TIA/EIA-485-A, the telecommunication industry’s most widely

used transmission line standard, describes the physical layer of

the RS-485 interface and is normally used with a higher-level

protocol, such as Profibus, Interbus, Modbus, or BACnet. This

allows for robust data transmission over relatively long distances.

The RS-422 physical layer is described in TIA/EIA-422-B. The

TIA/EIA-485-A standards are similar to those described in

TIA/EIA-422-B, and the values used to specify the drivers and

receivers in TIA/EIA-485-A standards are specified so that it

can meet both standards.

Rev. 0 | Page 1 of 12

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