Electrical devices such electrical motors and lighting systems could be controlled either manually or automatically. Traditionally, autmatic control has included switches and relay either electro mechanical or solid state. Switches has been used to start and energize these devices. The most modern way to control automatic electrical devices is the Programming Logic Control System or PLC.
A PLC system includes two main parts:Hardware and Software
PLC Hardware subsystem includes the following:
Input Interfaces
Output Interfaces
CPU(Central Processing Units)
Memories
Power Supply
Communications Ports
Input Interface
These are electronic cards used to collect information from the external world; and, it transfers this information to the Central Processing Unit(CPU). These information coming from external world could be analog data or digital data.
Aanalog Data could take many values in a second. For example the 120 VAC is taken many values in one second.
Digital Data could take could take just one value at the time either 0 or 1.
Output Interfaces
These are electronic boards used by the CPU to send data to the external world either to turn an electrical devices or to turned off.
The CPU(Central Processing Unit)
It is a computer brain that is able to process data a very high speed. It makes decisions based on the comparison of data that it gets from input Interfaces and it sends the results of these comparisons to the output interfaces to control electrical devices. CPU is able to deals just with digital data.
Memories
ROM(Read Only Memory) It is used by the CPU just read data. Normally ROM has the CPU machine language.
RAM(Random Access Memory) this type of memory is used by the CPU for data processing.
Video PLC Hardware