major units of a plc

MAJOR UNITS OF A PLC

A PLC consists of the three major elements, as shown in Fig.15.17. They are :

1. Central processing unit (CPU),

2. Programmer/monitor (PM), and

3. Input/output module (I/O).

1. Central Processing Unit (CPU)

• The CPU controls and processes all the operations within the PLC, that's why this unit is referred as the 'brain' of the PLC.

• Function: The CPU (i) receives input data from various sensing devices such as switches, (ii) executes the stored program, and (iii) delivers corresponding output signals to various load control devices such as relay coils and solenoids.

• It consists a microprocessor with a fixed memory (ROM-‘read only memory') and a variable memory (RAM–‘random access memory').

2. Programmer/Monitor (PM)

• The programmer/monitor unit allows the user to enter the desired programme into the RAM.

• The programme which is entered in relay logic (in RAM) determines the sequence of operation of the system to be controlled.

3. Input/Output Module (1/0)

• This module interfaces between the fluid power system input sensing and output load devices and the CPU.

• Function: The purpose of the I/O module is to transform the various signals received from or sent to the fluid power interface devices (such as push-button switches, pressure switches, limit switches, solenoid coils, motor relay coils, and indicator lights).