PLC Primer.pdf.PDF

PLC

Primer

T ABLE OF C ONTENTS

What Is a PLC? pg 3

Why Use PLCs? pg 4

But What Exactly Is a PLC? pg 5

A Little More About Inputs and Outputs pg 7

And a Little More About the Control Program pg 10

So How Does a PLC Keep All This Straight?

pg 13

To Sum It All Up

pg 14

Want To Learn More?

pg 15

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A PLC Primer

W HAT

I S A

PLC?

A programmable logic controller , also called a PLC or program-

mable controller , is a computer-type device used to control equipment

in an industrial facility. The kinds of equipment that PLCs can control

are as varied as industrial facilities themselves. Conveyor systems,

food processing machinery, auto assembly lines…you name it and

there’s probably a PLC out there controlling it.

In a traditional industrial control system, all control devices are wired

directly to each other according to how the system is supposed to

operate. In a PLC system, however, the PLC replaces the wiring

between the devices. Thus, instead of being wired directly to each

other, all equipment is wired to the PLC. Then, the control program

inside the PLC provides the “wiring” connection between the devices.

The control program is the computer program stored in the PLC’s

memory that tells the PLC what’s supposed to be going on in the

system. The use of a PLC to provide the wiring connections between

system devices is called softwiring .

E XAMPLE

LetÕs say that a push button is supposed to control the operation of a

motor. In a traditional control system, the push button would be wired

directly to the motor. In a PLC system, however, both the push button

and the motor would be wired to the PLC instead. Then, the PLCÕs

control program would complete the electrical circuit between the two,

allowing the button to control the motor.

In a traditional system, all control devices

are wired directly to each otherÉ

ÉIn a PLC system, all control devices are

wired to the PLC.

PLC

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A PLC Primer

W HY

U SE

PLC S ?

The softwiring advantage provided by programmable controllers is

tremendous. In fact, it is one of the most important features of PLCs.

Softwiring makes changes in the control system easy and cheap. If

you want a device in a PLC system to behave differently or to control

a different process element, all you have to do is change the control

program. In a traditional system, making this type of change would

involve physically changing the wiring between the devices, a costly

and time-consuming endeavor.

E XAMPLE

LetÕs say that two push buttons, PB1 and PB2, are connected to a PLC.

Two pilot lights, PL1 and PL2, are also connected to the PLC. The way

these devices are connected now pressing push button PB1 turns on

pilot light PL1 and pressing push button PB2 turns on pilot light PL2.

LetÕs say that you want to change this around so that PB1 controls PL2

and PB2 controls PL1. In a traditional system, you would have to rewire

the circuit so that the wiring from the first push button goes to the sec-

ond pilot light and vice versa. However, because these devices are con-

nected to a PLC, making this change is as simple as making a small

change in the control program.

In addition to the programming flexibility we just mentioned, PLCs

offer other advantages over traditional control systems. These advan-

tages include:

• high reliability

• small space requirements

• computing capabilities

• reduced costs

• ability to withstand harsh environments

• expandability

Industrial Text & Video Company 1-800-752-8398 www.industrialtext.com

A PLC Primer

B UT

W HAT

E XACTLY

I S A

PLC?

A PLC basically consists of two elements:

• the central processing unit

• the input/output system

The Central Processing Unit

The central processing unit (CPU) is the part of a programmable

controller that retrieves, decodes, stores, and processes information.

It also executes the control program stored in the PLC’s memory. In

essence, the CPU is the “brains” of a programmable controller. It

functions much the same way the CPU of a regular computer does,

except that it uses special instructions and coding to perform its func-

tions. The CPU has three parts:

• the processor

• the memory system

• the power supply

The processor is the section of the CPU that codes, decodes, and

computes data. The memory system is the section of the CPU that

stores both the control program and data from the equipment con-

nected to the PLC. The power supply is the section that provides the

PLC with the voltage and current it needs to operate.

The CPU has three main partsÉ

Éthe processorÉ

Éthe memory systemÉ

Éand the power supply.

Processor

Memory

System

CPU

Supply

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Power

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