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 PLC Devices.
The signals used in RS-232 communication for PLC devices include:
| Signal | DTE Direction | Description |
|---|---|---|
| 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. |
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).
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. 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:
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.