How Plcs Process Programs - Mitsubishi Electric FX Series Training Manual

How PLCs Process Programs

How PLCs Process Programs
2.3
A PLC performs its tasks by executing a program that is usually developed outside the controller
and then transferred to the controller's program memory. Before you start programming it is use-
ful to have a basic understanding of how PLCs process these programs.
A PLC program consists of a sequence of instructions that control the functions of the controller.
The PLC executes these control instructions sequentially, i.e. one after another. The entire pro-
gram sequence is cyclical, which means that it is repeated in a continuous loop. The time
required for one program repetition is referred to as the program cycle time or period.
Process image processing
The program in the PLC is not executed directly on the inputs and outputs, but on a "process
image" of the inputs and outputs:
Input process image
At the beginning of each program cycle the system polls the signal states of the inputs and stores
them in a buffer, creating a "process image" of the inputs.
2 - 4
Input signals
Input terminals
Process image
of inputs
Process image
of outputs
Output terminals
Output signals
Switch on PLC
Delete output memory
Poll inputs and signal states
and save them in the process
image of the inputs
PLC program
Instruction 1
Instruction 2
Instruction 3
....
....
....
Instruction n
Transfer process image
to outputs
The Hardware
MITSUBISHI ELECTRIC