Mitsubishi Digital Electronics FX2NC Manual

A SERVICE OF

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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.