Nowadays, small computers make up remote terminal units or RTUs and SCADA units. Users program controller algorithms into these units, allowing them to control sensors and actuators. Likewise, they can program algorithms for logic solvers, power factor calculators, flow totalizers, and many more, according to actual requirements in the field.
Present RTUs are powerful computers able to solve complex algorithms or mathematical formula describing external functions. Sensing devices or sensors gather data from the field, sending the signals back to the RTU. By solving the algorithms in it using the input signals, the RTU then sends out control instructions to valves or other control actuators. As scan periods in RTUs are very small, the entire activity happens very fast, hardly taking a few milliseconds, with the RTU repeating the process.
Regulatory agencies certifying RTUs prefer use of dedicated hardware for solving certain safety related functions such as toxic gas concentration and smoke detection. Therefore, they make sure of the reliability of detection for safety related functions.
The RTU operates in a closed system. Sensors measure the process variables, while actuators adjust the process parameters and controllers solve algorithms for controlling the actuators in response to the measured variables. The entire system works together based on wiring or some form of communication protocol. This way, the RTU enables the field processes near it to operate according to design.
Before the controller in the RTU can solve the algorithm, it has to receive an input from the field sensor. This requires a defined form of communication between the RTU and the various sensors in the field. Likewise, after solving the algorithm, the RTU has to communicate with the different actuators in the field.
In practice, sensors usually feed into a master terminal unit or MTU that conditions their input, changing it to the binary form from the analog form, if necessary. This is because sensors may be analog or digital types. For instance, a switch acting as a sensor can send information about its state using a digital one or +5 V when it is open and a digital zero or 0 V when it is closed. However, a temperature sensor has to send an analog signal or a continuously varying voltage representing the current temperature.
The MTU uses analog to digital converters to convert analog signals from the sensors to a digital form. All communication between the MTU and the RTU is digital in nature, and a clock signal synchronizes the communication.
The industry uses RTUs as multipurpose devices for remote monitoring and control of various devices and systems, mostly for automation. Although industrial RTUs perform similar function as programmable logic circuits or PLCs do, the former operates at a higher level as RTUs are basically self-contained computer units, containing a processor and memory for storage. Therefore, the industry often uses RTUs as intelligent controllers or master controller units for controlling devices that automate a process. This process can be a part of an assembly line.
By monitoring the analog and digital parameters from the field through sensors and connected devices, RTUs can control them and send feedback to the central monitoring station for industries dealing with power, water, oil, and similar distribution.