RS-485 Communication with PLC Devices

Help Contents


RS-485 is a standard published by the EIA.

The RS-485 standard defines:

The remainder of this article describes how RS-485 is used by PLC and RTU Devices.

Electrical Requirements

Communication in RS-485 uses a pair of wires.

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

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

RS-485 communication uses the voltage difference between two wires. This provides immunity to noise and allows transmission distance up to 1500 metres.

Connector Type and Pin Layout

No common standard exists for RS-485 connectors. Many hardware devices implement an RS-485 connection using screw terminals with these labels:

Data+The positive signal of the transmit and receive pair
Data-The negative signal of the transmit and receive pair

Very few computers have a built-in serial RS-485 port. Common ways of providing an RS-485 serial ports include:

For information on how to use RS-485 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.