What is meant by RTU?
An RTU (Remote Terminal or Terminating Unit) is a microprocessor-controlled device that interfaces with objects and reports status or controls about the local condition of equipment. In the earliest days, RTUs were “bundled” with other equipment eg telecommunications equipment, generators, pumps etc, as a means to monitor status and health of its associated equipment. The next generation of RTUs extended control to operators to remotely start and stop devices using RTUs as the end to end communications platform. In these cases, the HMI could have been a bespoke software application or a touch panel many hundreds of kilometers away. In time as RTUs matured and became more like PLCs, engineers began using RTUs in place of PLCs because typically RTUs had superior communications capability over that of traditional PLCs.
RTUs have more recently become the pseudo control device for distributed asset management. It is very common to see water, gas and power assets managed and controlled using RTUs with PLCs being reserved for very high-performance application like asset protection. In these cases, it is common place to see PLCs and RTUs used together, ie the PLC with the role of chief controller, and the RTU as the marshaling communications device which incorporates all health and status reporting. This is most appropriate when the SCADA network protocol is DNP3 as most PLCs do not support this network protocol.
What are the common PLC & RTU Programming Languages?
Under the IEC 61131-3 standard, PLCs (and RTUs) can be programmed using standards-based programming languages. The most commonly used programming language is Ladder Diagram (LD) also known as relay ladder logic. It uses Contact-Coil logic to make programs like an electrical line diagram. The full list of programming styles are as follows:
- Ladder diagram (LD), graphical
- Function block diagram (FBD), graphical
- Sequential function chart (SFC), has elements to organize programs for sequential and parallel control processing.
- Structured text (ST), textual
There is no “one” philosophy that fits all situations. Sometimes SFC is the simplest method when developing a new concept. To the contrary when editing large arrays of points, STs are more suitable. The power of the debug tools that are made available with the software platform also may influence what the engineer may choose to implement the software with.
The most important aspect of which PLC tool to select is maintainability for parties other than the original developer. The original developers often fully understand the system design concepts and may even be a primary contributor to the design. The maintainer may not have the luxury of understanding the system design. For this reason, systems design should consider the use of tools for development and also the maintenance of the system.