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ACOE345 Introduction to PLCs 1

• What is a PLC?

Introduction to Programmable Logic Controllers (PLCs)

• Hardware components of a PLC

• Memory structure of a PLC

• PLC Modes of Operation

• Wiring Diagram

• Digital Inputs/Outputs


What is a PLC?

• A Programmable Logic Controller (PLC) is– a microprocessor based control system,

– designed for use in an industrial environment,– programmed to sense, activate and control industrial equipment.

ABC ABC

2

1 3

Size

Start

Stop Run Empty Done

Control Panel

PLC Controlled System


Hardware components of a PLC

Inte

rna

l Bu

s

CPU

ROM

RAM

EAPROM

Programming Port

Digital InputInterface

Digital OutputInterface

Pulse Counterand Timer

Analog InputOutput Interface

CommunicationInterface

Additional DigitalInput/Output

Interface

Computer

Programming Console

Hand-held Terminal

DC Voltage Input

AC Voltage Input

Relay Output

Transistor Output

Triac Output

PLC


Memory structure of a PLC

The operating system of a PLC operates in one of two modes: theProgramming mode and the Run mode.In the Programming mode it communicates with the programming device(hand-held terminal, programming console or PC) to download the user'sprogram in the user's program space.In the Run mode it executes the instructions stored in the user's program area.

Operating System

Memory MappedInput/Output

System Data

User's ProgramSpace

ROM

RAM

EAPROMor FlushEPROM

Memory area that holds an image of the state of the inputs as well as thevalues to ne to be output.

Memory area used by the operating system to store system data

Memory area that holds the instructions of the user's program.


PLC Mode of Operation

• A PLC operates in one of two modes:– Program Mode: Download the user’s program from

the programming device to the memory of the PLC.– Run Mode: Execute the user’s program using the

scan-based execution model, in three phases:

• Read the state of the inputs and store them in the RAM referred to as the input image memory..

• Execute the user’s program and update the state of the outputs in the RAM (output image memory).

• Copy the states of the outputs from the RAM to the physical output devices.

– The time needed to complete one scan cycle depends on the length and complexity of the user’s program.

– PLC manufacturers specify the maximum number of program steps (say 1000) and the average scan time (few ms).

ModeControl

Scan inputsand store states

in RAM

Execute user'sprogram

Update outputs

Run Mode

Program Mode

1 Scan Cycle


Wiring Diagram

O:1/00

I:1/00

I:1/01

I:1/02

I:1/03

I:1/04

I:1/05

I:1/06

I:1/07

O:1/01

O:1/02

O:1/03

O:1/04

O:1/05

O:1/06

O:1/07

Com

PLC

M

Item Pusher

Belt Motor

Run

Empty

Done

Start

Stop

Size Selector

Empty Sensor

Item Sensor


DC Voltage Digital Input

• The optoisolator isolates the internal components of the PLC and the external input devices.

• Diode D1 provides protection during switching from inductive devices.

• When the switch is closed, current flows through the status LED and the opto-isolator, giving a logic 1 to the input port.

24Vdc

Status LED

To Input Port

D1R1

R2

Input Device

Terminal Connections on PLC

DC Voltage Digital Input:- Sink Input Circuit

24Vdc

Status LED

To Input Port

D1R1

R2

Input Device


DC Voltage Digital Input:- Source Input Circuit


Connecting DC Proximity Switches

24Vdc

Status LED

To InputPort

D1R1

R2ProximitySwitch


Connecting an NPN Proximity Switch

24Vdc

Status LED

To InputPort

D1R1

R2


Connecting a PNP Proximity Switch

ProximitySwitch


AC Voltage Digital Input• AC input devices switch On and Off the

mains voltage.• The mains voltage must be reduced using a

voltage divider. • The mains voltage must be converted to DC

using a rectifier and a smoothing circuit.

• An AC proximity switch switches On and Off the mains voltage by triggering (firing) a triac. The proximity sensor controls the gate current of the triac.

240Vac Status LED

To InputPort

Inp

ut D

evi

ce

Term

ina

l Co

nn

ect

ion

s o

n P

LC

AC Voltage Digital Input Circuit

240Vac Status LED

To InputPort

Term

inal

Con

nect

ions

on

PLC

Connecting an AC Proximity Switch

ProximitySwitch


Digital Output (Relay and Transistor)

• Most PLCs have relay outputs.

• Relay outputs have the advantage that they can control DC as well as AC loads at relatively high currents.

• The disadvantages of relays are due to their mechanical contacts.

• Transistor outputs provide higher switching speeds with more switching life.

• Transistor outputs can only be used with DC loads and are polarity sensitive. A diode is usually used for reverse polarity protection.

• Another diode is usually used to protect the output transistor when switching inductive loads.

Status LED

FromOutput

Port

Relay Output Circuit

+Vdc

Load

AC or DCSupply

Status LED

FromOutput

Port

NPN Transistor Output Circuit

+Vdc

Load


Digital Output Using Triacs

• Triac outputs are used to avoid the problems due to the mechanical contacts of relays.

• An opto-diac is used to fire the triac.

• An RC circuit is usually used to avoid problems due to the phase difference between the voltage and the current when driving inductive loads.

AC SupplyStatus LED

FromOutput

Port

Triac Output Circuit

Load

Education

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