Functions of CPU in PLC

Like other computerized devices, there is a Central Processing Unit (CPU) in a PLC. The CPU, which is the “brain” of a PLC, does the following operations:

Updating inputs and outputs

This function allows a PLC to read the status of its input terminals and energize or deenergize its output terminals.

Performing logic and arithmetic operations

A CPU conducts all the mathematic and logic operations involved in a PLC.

Communicating with memory

The PLC’s programs and data are stored in memory. When a PLC is operating, its CPU may read or change the contents of memory locations.

Scanning application programs

An application program, which is called a ladder logic program, is a set of instructions written by a PLC programmer. The scanning function allows the PLC to execute the application program as specified by the programmer.

Communicating with a programming terminal.

The CPU transfers program and data between itself and the programming terminal.

A PLC’s CPU is controlled by operating system software. The operating system software is a group of supervisory programs that are loaded and stored permanently in the PLC’s memory by the PLC manufacturer.