SOLVE

To understand the basic working of Programmable Logic Controller (PLC) by taking a simple program.

STEP 1

Create a program (ladder diagram) to turn on the lamp only when both the switches are closed (And operation).

I₁ I₂ O₁

This ladder diagram represents an And operation

STEP 2

Download the program (ladder diagram) developed into PLC's memory using PROGRAMMER.

Handheld programmer, PC or industrial computer programming terminal can be used as the PROGRAMMER

The processor must be switched to program mode before downloading the program into PLC's memory

In program mode, all outputs from the PLC are forced OFF regardless of their rung logic status, and the ladder I/O scan sequence is halted

Program mode

Downloading...

HALTED (Forced off)

STEP 3

After downloading the program, put the PLC into RUN mode to solve the programmed instructions.

In RUN mode, the program will continuously run and solve the programmed instructions

The process of solving the programmed instruction is sometimes called as solving the logic. This constant running of the program in a PLC is called scanning

Or, in other words, in RUN mode, the user program is executed. Input devices are monitored and output devices are energized accordingly

RUN mode...

OFF