PLCs and PCs

When PLCs were introduced over 30 years ago, relay ladder logic (RLL) was the standard programming language. Ladder diagrams identify the order of operation of devices. The logic can be rewired by programming on the CRT display and tested, without any physical rewiring of the banks of relays. The displays show a ladder diagram with contacts, coils and circuit connection very similar to a schematic diagram. RLL was not designed to support a wide range of control functions.

The main input devices to PLCs are push-buttons, limit switches and other command devices. The output devices are contactors, relays, solenoids and indicating lamps.

PLCs have been moving towards commodity hardware items, as PCs already have, and the trend is towards providing more complete application solutions. This means more open systems and commercial off-the-shelf technologies to meet user demands for performance, ease of use and lower cost of ownership.

Continued optimization of overall plant performance requires leveraging plant information with automation applications. Even PLC architectures are increasingly being dominated by the flow of information. The use of object-based technologies, industrial Ethernet and Internet access to information are examples of this trend. PLCs along with PCs are changing from closed-control-specific devices to open information servers providing access to previously untapped operational information.

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