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.
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.
No common standard exists for RS-485 connectors. Many hardware devices implement an RS-485 connection using screw terminals with these labels:
| Connection | Description |
|---|---|
| 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:
To learn how data is sent serially.
To learn about configuring serial port mode strings in Fernhill SCADA.
To learn about the editor that can edit serial port mode strings.