Programmable Logic Controllers; What Is A Plc - Mitsubishi MELSEC System Q Manual

Programmable Logic Controllers

2 Programmable Logic Controllers
2.1

What is a PLC?

In contrast to conventional controllers with functions determined by their physical wiring the
functions of programmable logic controllers or PLCs are defined by a program. PLCs also have
to be connected to the outside world with cables, but the contents of their program memory can
be changed at any time to adapt their programs to different control tasks.
Programmable logic controllers input data, process it and then output the results. This process
is performed in three stages:
b an input stage,
b a processing stage
and
b an output stage
The input stage
The input stage passes control signals from switches, buttons or sensors on to the processing
stage.
The signals from these components are generated as part of the control process and are fed to
the inputs as logical states. The input stage passes them on to the processing stage in a
pre-processed format.
The processing stage
In the processing stage the pre-processed signals from the input stage are processed and
combined with the help of logical operations and other functions. The program memory of the
processing stage is fully programmable. The processing sequence can be changed at any time
by modifying or replacing the stored program.
The output stage
The results of the processing of the input signals by the program are fed to the output stage
where they control connected switchable elements such as contactors, signal lamps, solenoid
valves and so on.
MELSEC System Q Beginners Manual
Programmable Logic Controller
Input
Switch
Input Stage
Output
Processing Stage Output Stage
What is a PLC?
Contactors
2 – 1