Remote terminal unit

Since RTUs may be routinely deployed in pipeline and grid guarding systems, or in other hard-to-reach or extreme environments (for example in the Biosphere 2 project), they are required to operate under harsh conditions, and implement energy-saving measures (such as switching off IO modules when not in use).

A form of power supply will be included for operation from the AC mains for various CPU, status wetting voltages and other interface cards.

This is usually accomplished by using an isolated voltage or current source to sense the position of a remote contact (open or closed) at the RTU site.

While not as commonly used, analog outputs may be included to control devices that require varying quantities, such as graphic recording instruments (strip charts).

Summed or processed data quantities may be generated in a master SCADA system and output for display locally or remotely, wherever needed.

Modern RTUs are usually capable of executing simple programs autonomously without involving the host computers of the DCS or SCADA system to simplify deployment and to provide redundancy for safety reasons.

An RTU may be interfaced to multiple master stations and IEDs (Intelligent Electronic Devices) with different communication protocols (usually serial (RS-232, RS-485, RS-422) or Ethernet).

Digital (status) values observe a similar technique and only transmit groups (bytes) when one included point (bit) changes.

Multiple RTUs or IEDs may share a communications line, in a multi-drop scheme, as units are addressed uniquely and only respond to their own polls and commands.