The processors receive information from input modules, process the information and decide control actions to be performed by the output modules. The input modules receive information from sensing instruments in the process (or field) and the output modules transmit instructions to the final control elements, such as control valves
The field inputs and outputs can either be continuously changing analog signals
In 1993, with the release of IEC-1131, later to become ...
Quite complex systems can be created with networks of these controllers communicating using industry standard protocols. Networking allow the use of local or remote SCADA operator interfaces, and enables the cascading and interlocking of controllers. However, as the number of ...
The simplest control systems are based around small discrete controllers with a single control ...
Industrial control system (ICS) is a general term that encompasses several types of control systems Such systems can range in size from a few modular panel-mounted controllers to large interconnected and interactive distributed control systems with many thousands of field ...
In more recent years,[when?] small products called programmable logic relays (PLRs) or smart relays, have become more common and accepted. These are similar to PLCs and are used in light industry where only a few points of I/O are needed, and low cost is desired. These small ...
Very complex process control, such as used in the chemical industry, may require algorithms and performance beyond the capability of even high-performance PLCs. Very high-speed or precision controls may also require customized solutions; for example, aircraft flight controls. ...
Some PLCs enforce a strict left-to-right, top-to-bottom execution order for evaluating the rung logic. This is different from electro-mechanical relay contacts, which in a sufficiently complex circuit may either pass current left-to-right or right-to-left, depending on the ...
This is a programming example in ladder diagram As an example, say a facility needs to store water in a tank. The water is drawn from the tank by another system, as needed, and our example system must manage the water level in the tank by controlling the valve that refills the ...