RS-232 Communication with PLC Devices

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Introduction

RS-232 is a communication standard published by the EIA.

The RS-232 standard defines:

In PLC Device communications, RS-232 is used to create a point to point link.

The remainder of this article describes how RS-232 is used by typical PLC Devices.

RS-232 Signals

The typical signals used in RS-232 communication for PLC devices include:

SignalDTE DirectionDescription
TxD Out Transmitted data bits.
RxD In Received data bits.
DTR Out Indicates the DTE device is ready to receive data.
DCD In Data carrier detect.
DSR In Data set ready.
RTS Out Indicates the DTE device needs to send data.
CTS In Tells the DTE device it can send data.

Electrical Requirements

Each signal in RS-232 is represented by a single wire.

The voltage of the signal wire compared with the Common Signal Ground indicates the state of the signal:

Signal Transmitter Receiver
Mark (1) or Not Asserted -5v to -15v -3v to -15v
Space (0) or Asserted +5v to +15v +3v to +15v

RS-232 communication is single ended. This limits the noise immunity and transmission distance. The RS-232 standard recommends a maximum cable length of 15 metres (50 feet).

Connector Type and Pin Layout

The RS-232 standard states connections should use a D-subminiature 25 pin (DB-25) connector.

A few very old PLC devices follow the RS-232 standard. However most PLC devices use a smaller connector, for example a DE-9 or a mini-DIN connector. The style of connector and pin assignment varies between different PLC manufacturers.

For information on how to connect a PLC Device to Fernhill SCADA, using an RS-232 link, 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.