Architecture and communications

An RTU monitors the field digital and analog parameters and transmits all the data to the Central Monitoring Station. An RTU can be interfaced with the Central Station with different communication media (usually serial (RS232, RS485, RS422) or Ethernet). RTU can support standard protocols (Modbus, IEC 60870-5-101/103/104, DNP3, ICCP, etc.) to interface any third party software. In some control applications, RTUs drive high current capacity relays to a digital output (or "DO") board to switch power on and off to devices in the field. The DO board switches voltage to the coil in the relay, which closes the high current contacts, which completes the power circuit to the device. An RTU can monitor analog inputs of different types including 4 to 20 milliamperes (4–20 mA), 0 to 10 V., −2.5V to 2.5V, 1 to 5V etc.; the RTU or host system then translates this raw data into the appropriate units such as gallons of water left or temperature before presenting the data to the user via the HMI or MMI.

RTUs differ from Programmable Logic Controllers (PLCs) in that RTUs are more suitable for wide geographical telemetry, often using wireless communications, while PLCs are more suitable for local area control (plants, production lines, etc.) where the system utilizes physical media for control. The IEC 61131 programming tool is more popular for use with PLCs, while RTUs often use proprietary programming tools