Remote Terminal Units (RTUs)

Remote terminal units (RTUs) collect data automatically and connect directly to sensors, meters, loggers or process equipment. They serve as slave units to supervisory controllers or supervisory control and data acquisition (SCADA) masters. Remote terminal units are located near the monitored process and transfer data to the controller unit on command. They often include integral software, data logging capabilities, a real-time clock (RTC) or totalizer, and a battery backup. Intrinsically safe remote terminal units are designed to operate safely in hazardous environments. Devices with weather tight enclosures are designed to prevent the ingress of moisture, dust or other environmental contaminants. Closed loop systems use proportional, integral and derivative (PID) control; proportional and integral (PI) control; proportional and derivative (PD) control; or proportional (P) control. Redundant RTUs are complete remote terminal units that contain all of the transceivers, encoders, and processors needed for proper functioning in the event that a primary RTU stops working.

Important specifications for remote terminal units (RTUs) include communication type, number of ports, and memory size. Serial devices use communication protocols such as RS232, RS485, and RS422. Networked products often use Ethernet, a local area network (LAN) protocol that uses a bus or star typology and supports data transfer rates of 10 Mbps. Remote terminal units that use radio, video, telephone, or current loop communications are also available. Fiber optic devices use optical cables to transmit light signals over long distances. Web-enabled RTUs use the Internet for communications. Programmable logic controllers (PLC) can be used as stand-alone devices or in conjunction with a SCADA or other control system. In terms of ports, most suppliers specify the number of analog or digital input/output (I/O) ports. Memory size is usually measured in megabytes (Mb).

Remote terminal units differ in terms of features. Devices that include alarms, buzzers, or visual indicators such as blinking lights alert operators about various conditions. Auto-dialing RTUs automatically call a non-dedicated phone number whenever data needs to be transferred. Differential inputs eliminate electrical noise from small amplitude signals. Isolated inputs convert electrical signal inputs into optical signals which are then converted back to electrical signals. Remote terminal units with an expansion card provide additional data storage or processing power. Devices that are designed for environmental monitoring check weather conditions or indoor air quality. Some remote terminal units are suitable for indoor use and mount in standard 19” telecommunications racks. Others are designed for process monitoring in applications such as oil field exploration.

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