Programmable Logic Controllers (PLCs) are integrated as either single or modular units.
An integrated or Compact PLC is built by several modules within a single case. Therefore, the I/O capabilities are decided by the manufacturer, but not by the user. Some of the integrated PLCs allow to connect additional I/Os to make them somewhat modular.
A modular PLC is built with several components that are plugged into a common rack or bus with extendable I/O capabilities. It contains power supply module, CPU and other I/O modules that are plugged together in the same rack, which are from same manufacturers or from other manufacturers. These modular PLCs come in different sizes with variable power supply, computing capabilities, I/O connectivity, etc.
Modular PLCs are further divided into small, medium and large PLCs based on the program memory size and the number of I/O features.
Small PLC is a mini-sized PLC that is designed as compact and robust unit mounted or placed beside the equipment to be controlled. This type of PLC is used for replacing hard-wired relay logics, counters, timers, etc. This PLC I/O module expandability is limited for one or two modules and it uses logic instruction list or relay ladder language as programming language.
Medium-sized PLC is mostly used PLC in industries which allows many plug-in modules that are mounted on backplane of the system. Some hundreds of input/ output points are provided by adding additional I/O cards – and, in addition to these – communication module facilities are provided by this PLC.
Large PLCs are used wherein complex process control functions are required. These PLCs’ capacities are quite higher than the medium PLCs in terms of memory, programming languages, I/O points, and communication modules, and so on. Mostly, these PLCs are used in supervisory control and data acquisition (SCADA) systems, larger plants, distributed control systems, etc.
Some of the manufacturers or types of PLCs are given below:
- Allen Bradley PLCs (AB)
- ABB PLCs (Asea Brown Boveri)
- Siemens PLCs
- Omron PLCs
- Mitsubishi PLCs
- Hitachi PLCs
- Delta PLCs
- General Electric (GE) PLCs
- Honeywell PLCs
- Modicon PLCs
- Schneider Electric PLCs
- Bosch PLCs