Power supply: It converts power line voltage to those required by the components.

Input systems: Input signals for the PLC are the process variables. These signals can be analog or digital, low or high frequency, maintained or momentary action. Typically they are presented as a varying voltage, current, or resistance value. Signals from thermocouples & resistance temp. Detectors are examples of analog signals. Some flow meters & strain gauges provide variable frequency signals while pushbuttons, limit switches or relay are examples of digital signals. This system transfer the data to the CPU.

Outputs:
There are three common categories of outputs: Discrete, Register & Analog
Discrete outputs can be pilot lights, solenoid valves, or annunciator windows,
Register outputs can drive signals to panel meters or display.
Analog outputs can drive variable speed drives or to l/P convectors.

Central Processing Unit: The central processing unit performs the tasks necessary to fulfill the PLC function. Among these tasks are scanning, I/O bus traffic control, program execution, peripheral & external device communications, data handling execution.

Memory Unit : The Memory Unit serves several functions . It is the library where the application program, PLC's executive program is stored. An executive program functions as the operating system of PLC. It is the program that interprets, manages, & executes the user's application program. Finally the Memory Unit is the part of PLC where process data from the input modules & control data for output modules are temporarily stored as data tables. These data tables are used by the CPU and, when appropriate, sent to the output modules.

Memory Range: Small PLC: 256 to 2K words Medium PLC: 2K to 12K words Large PLC: 12K & above

Programmer Units: The programmer unit provides an interface between the PLC and user during program development, start-up, & troubleshooting.

PLC' s are designed to operate near the equipment they are meant to control This means that they functions in hot, humid, noisy& dusty industrial environments.
Advantages of PLC:
1) Ease of programming & reprogramming in the plant.
2) A programming that is based on symbols familiar to most plant electrical personal.
3) High' reliability & minimum maintenance.
4) Small physical size.
5) Ability to communicate with computer systems in the plant.
6) Low initial investment.
7) Rugged construction.
8) Modular design. It consists of a power supply, central processing unit, memory & program and peripheral device subsections.