Control Rookie Question - Terminology


Thread Starter


First off, hello! I'm just getting into the paper making industry and my head is spinning when it comes to process control. My background is electronics and I had no automation background before this job.
Simple questions to get on track if you could be so helpful.... how do these terms relate to process control and to each other?

-Field Controller


Programmable Logic Controller. A programmable device that connects to sensors and actuators and controls a machine or process. Designed to replace cabinets of physical relays, the original programming paradigm was derived from the design drawings for those relay systems, and is called "ladder logic" after the way the diagrams look. Current PLC's support a spectrum of languages, often with the option to mix and match within the same program. In addition to ladder, some support a variation on assembler for their processor called statement list, more BASIC-like languages sometimes called structured text, graphical languages for design of machine state handling or emulation of boolean electronic circuits, and so on.


Originally developed by MODicon (IIRC the name stands for "MODular Industrial CONtroller), the first PLC company, as a protocol to talk to their PLC's, it's now supported by thousands of automation and instrumentation products from many many vendors. In addition to the original serial version, which came in two versions where data was transmitted either in raw digital form or as ASCII (for use over modems), there is now an ethernet version called Modbus/TCP. The protocol is open, both speech AND beer, and easy to write a driver for, unlike the majority of currently popular industrial protocols.


Originally stood for Ole for Process Control, now stands for something else. Essentially provides a layer of indirection so the control software on your PC doesn't have to delve too much into the details of the PLC system it's connected to. You install the OPC server software which is PLC-specific and takes care of understanding the PLC comm protocol and communicating directly with it. Then your software (the "client") can interface to the server on a more or less generic basis regardless of which PLC is on the other side.