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PLCs > PLCs - Pg. 90

90 CHAPTER 5 How Industrial Networks Operate operating as slave nodes, and are controlled via an upstream RTU or PLC. As with all technology, IEDs are growing more and more sophisticated over time, and an IED may perform other tasks, blurring the line between device types. However, to simplify things for the purposes of this book, an IED can be considered to support a specific function (i.e., a motor can spin at different frequencies) within the control system, typically within a specific control loop, whereas RTUs and PLCs are designed for general use (i.e., they can be programmed to control the speed of a motor, to engage a lock, to activate a pump, etc.). RTUs A Remote Terminal Unit (RTU) typically resides in a substation or other remote location. RTUs monitor field parameters and transmit that data back to a central monitoring station--typically either a Master Terminal Unit (MTU), or a centrally located PLC, or directly to an HMI system. RTUs, therefore, include remote com- munications capabilities, consisting of a modem, cellular data connection, radio, or other wide area communication capability. They will typically use industrial net- work protocols such as DNP3 to communicate between master and remote units, and either DNP3 or Modbus, Profibus or some other common fieldbus protocol to communicate with IEDs (see Chapter 4, "Industrial Network Protocols"). RTUs and PLCs continue to overlap in capability and functionality, with many RTUs integrating programmable logic and control functions, to the point where an RTU can be thought of as a remote PLC. PLCs A programmable logic controller (PLC) is a specialized computer used to automate functions within industrial networks. Unlike desktop computers, PLCs are typically materially hardened (making them suitable for deployment on a production floor) and may be specialized for specific industrial uses with multiple specialized inputs and outputs. PLCs also differ from desktop computers in that they do not typically use a commercially available operating system (OS); instead they rely on blocks of logic code that allow the PLC to function automatically to specific inputs (e.g., from sensors) with as little overhead as possible. PLCs were originally designed to replace relays, and very simple PLCs may be referred to as programmable logic relays (PLRs). PLCs typically control real-time processes, and so they are designed for sim- ple efficiency. For example, in plastic manufacturing, a catalyst may need to be injected into a vat when the temperature reaches a very specific value; if processing overhead or other latency introduces delay in the execution of the PLC's logic, it would be very difficult to precisely time the injections, which could result in quality issues. For this reason, the logic used on PLCs is typically very simple and usually based on ladder logic (although almost any programming language could theoreti- cally be supported). Again, as technology evolves, the line blurs between RTU, PLC, and IED, as can be seen in Emerson Process Management's ROC800L liquid hydrocarbon