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Introduction 1
1-2
1.2 What is a Programmable Controller?
A Pro
g
rammable Lo
g
ic Controller (PLC or pro
g
rammable controller) is a device that a user can
pro
g
ram to perform a series or sequence of events. These events are tri
gg
ered b
y
stimuli
(usuall
y
called inputs) received at the PLC or throu
g
h dela
y
ed actions such as time dela
y
s or
counted occur-rences. Once an event tri
gg
ers, it actuates in the outside world b
y
switchin
g
ON
or OFF electronic control
g
ear or the ph
y
sical actuation of devices. A pro
g
rammable controller
will continuall
y
‘loop’ throu
g
h its internal ‘user defined’ pro
g
ram waitin
g
for inputs and
g
ivin
g
outputs at the pro
g
rammed specific times.
Note on terminolo
gy
:
The term pro
g
rammable controller is a
g
eneric word used to brin
g
all the elements makin
g
the
control s
y
stem under one descriptive name. Sometimes en
g
ineers use the term
‘Pro
g
rammable Lo
g
ic Controller’, ‘PLC’ or ‘pro
g
rammable controller’ to describe the same
control s
y
stem.
The construction of a pro
g
rammable controller can be broken down into component parts. The
element where the pro
g
ram is loaded, stored and processed is often known as the Main
Processin
g
Unit or MPU. Other terms commonl
y
heard to describe this device are ‘base unit’,
‘controller’ and ‘CPU’. The term CPU is a little misleadin
g
as toda
y
s more advanced products
ma
y
contain local CPU devices. A Main CPU (or more correctl
y
a Main Processin
g
Unit)
controls these local CPUs throu
g
h a communication network or bus.
1.3 What do You Need to Program a PLC?
A variet
y
of tools are available to pro
g
ram the Mitsubishi FX famil
y
of PLCs. Each of these
tools can use and access the instructions and devices listed in this manual for the identified
PLC.