Modem use in SCADA systems

Often in SCADA systems the RTU (remote terminal unit (PLC, DCS or IED)) is located at a remote location. This distance can vary from tens of meters to thousands of kilometers. One of the most cost-effective ways of communicating with the RTU over long distances can be by dialup telephone connection. With this system the devices needed are a PC, two dialup modems and the RTU (assuming that the RTU has a built in COM port). The modems are put in the auto-answer mode and the RTU can dial into the PC or the PC can dial the RTU. The software to do this is readily available from RTU manufacturers. The modems can be bought off the shelf at the local computer store.

Line modems are used to connect RTUs to a network over a pair of wires. These systems are usually fairly short (up to 1 kilometer) and use FSK (frequency shift keying) to communicate. Line modems are used to communicate to RTUs when RS-232 or RS- 485 communication systems are not practical. The bit rates used in this type of system are usually slow, 1200 to 9600 bps.