RS-422 Communication with PLC Devices

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Introduction

RS-422 is a standard published by the EIA.

The RS-422 standard defines:

Note: The RS-422 standard does not define the number of circuits in a logical connection.

In the context of PLC Device communications the term RS-422 often implies two circuits :

The remainder of this article describes how RS-422 is used by typical PLC Devices.

Electrical Requirements

Each signal in RS-422 is represented by a pair of wires.

The difference in voltage of the pair of wires indicates the state of the signal:

Signal VA - VB
Mark (1) or Not Asserted -0.3v to -6v
Space (0) or Asserted +0.3v to +6v

Note: The V11 standard uses the labels A and B to identify each wire in the pair of wires.

Using the voltage difference between the two wires provides good immunity to electrical noise. It allows transmission distance up to 1500 metres.

One of the problems for anyone working with RS-422 is the labels used by different manufacturers are not consistent. Problems you might find include:

Connector Type and Pin Layout

No common standard exists for RS-422 connectors. Many hardware devices implement an RS-422 connection using screw terminals with these labels:

ConnectionDescription
Tx+The positive signal of the transmit pair
Tx-The negative signal of the transmit pair
Rx+The positive signal of the receive pair
Rx-The negative signal of the receive pair

Very few computers have a built-in serial RS-422 port. Common ways of providing an RS-422 serial ports include:

For information on how to use RS-422 adapters see the following guides:

Further Information

Serial Communications

To learn how data is sent serially.

Serial Port Mode String

To learn about configuring serial port mode strings in Fernhill SCADA.

Serial Port Mode Editor

To learn about the editor that can edit serial port mode strings.