Basic Architecture Of PLC

BLOCK DIAGRAM

Input module

Power supply

Memory

CPUOutput module

Programming unit

M

S

What is a PLC? …. slide1

A PLC is an Industrial Computer. Just like our Personal Computer it has Inputs and Outputs.

Some of the Inputs to the PLC are:Limit Switches.Float Switches.Push Buttons.Photoelectric Sensors.Pressure Switches.

Temperature Sensors.Pressure Sensors.

Discrete input control field devices

Analog input control field devices

Some of the Outputs to the PLC are: Motor Starters Pilot Lights Solenoids.

Formal Definition of PLC.

The formal definition of PLC comes from the National Electric Manufacturers Association (NEMA).

“A PLC [Programmable Logic Controller] is a digitally operated electronic system, designed for use in an industrial environment, which uses a programmable memory for the internal storage of user-oriented instructions for implementing specific functions such as

Logic Sequencing Timing Counting & Arithmetic…………………

to control, through digital or analog inputs and outputs, various types of machines or processes.

Fig-2 shows how a human being is involved with the PC.

How a Human Being is Involved with PLC?

PLC Hardware Configuration: 2 Types.

1] Fixed/ Composite PLC 2] Modular PLC

Allen Bradley SLC 500 Modular PLC

Power supply Processor Input modules Output module

Slot 0 Slot 2Slot 1 Slot 3

Interfacing of the PLC with Programming Units.

Personal Computer

RS 232

1747- PIC

1747- C10 cable

SLC – 500 PLC ( Modular )

Output modulePower supply Processor

Slot 0 Slot 2Slot 1 Slot 3

Input modules

1 2 3 4 5 6 C7 1110980 12 13

0 1 2 3 4 5 6 7 8 9

Power

RunForceCommunication

MicroLogix 1200

OUT

IN

Allen Bradley MicroLogix 1200 Fixed PLC

24 Volts

Power Supply

Motor

Starter

L2

PLC InputSection

PLC O/PSection

Sensor input to PLC

PLC Output to Motor Starter

Sensor

Input Module

24 VDC Power Supply

+

-

0

12

3

4

6

5

C

7

Input signal from limit switch to input screw terminal 0 on a 24 VDCinput module.

Just take a glance at the input module wiring with limit switch!!!

What Do You Mean By Sinking & Sourcing?

Solid State input devices with NPN transistors.Sinking Input Devices

Sourcing Input DevicesSolid State input devices with PNP transistors.

Sinking & Sourcing are the terms used to describe current flow through a field device in relation to the Power Supply and the associated I/O points

What Do You Mean By Sinking & Sourcing?

Input Module

24 VDCPowerSupply

-

+

Sinking Switch

Sourcing ModuleInput Module

Sourcing Switch

Sinking Module

24 VDCPowerSupply

+

-

Wiring Diagrams of AB SLC 500 1746-IV8 ………Sourcing I/P.

Input Module

01

2

3

4

6

5

C

7

- 24 VDC

+ 24 VDC

Input Module

24 VDCPowerSupply

-

+

Sinking Switch

Sourcing Module

Input Module

01

2

3

4

6

5

C

7

+ 24 VDC

- 24 VDC

Wiring Diagrams of AB SLC 500 1746-IB8 ………Sinking I/ P.

Input Module

Sourcing Switch

Sinking Module

24 VDCPowerSupply

+

-

Input Modules: Classification…………..

Discrete Input modules Analog Input Modules

Discrete AC input modules.

Discrete DC input modules.

Input Modules

The main Function of the Input module is…………

To take the field device Input signal.

Electrically isolate it. and

Send by the way of backplane board to the CPU.

Discrete DC Input Modules.

The most common module types available are listed below

24 VDC48 VDC10 - 60 VDC

120 VDC 230 VDC Sink/ Source 5 - 50 VDC

5 VDC TTL level5/12 VDC TTL level

Block Diagram of typical Discrete DC Input Circuit.

Module OperationDC input

Optical Isolation

Noise &debounce

Filter.

ThresholdDetector.

Logic

CPUInputStatus

File

LED

1 2 3

4

56

7

8

Simplified Circuitry of a DC Input Module

Discrete AC Input Modules.

The most common module types available are listed below

24 VAC48 VAC

120 VAC 240 VAC Nonvoltage

120 Volts Isolated 240 Volts Isolated

24 VAC/DC

Block Diagram Of Typical Discrete AC Input Circuit

Noise and

debounce filter

Bridge rectifier

Optical isolation

Input status table

Logic CPUThreshold detector

LED

To PC Internal sys. 1

AC

2

3Pulsating DC

4

5Filtered DC

6 7 8 10

9

11

OPTICAL Isolator is used to isolate high voltage incoming signals from the CPU’s lowervoltage levels. It protects the low voltage CPU and its associated circuitry by preventingspikes or high – voltage transients on the input circuit from transferring into the low voltagecircuitry.

Power file Conversion CircuitOptical Isolation CircuitLogic Circuit

Two Types of Discrete Input Device………….

Mechanical Devices Electronic Devices Limit Switches.

Toggle Switches

Selector Switches

Push Button

Contacts from an Electro-Mechanical relay. etc

Solid - State sensors like

Inductive Proximity device.

Capacitive Proximity device.Etc.

Binary Concept & Binary Data Representation……… slide1

Bit Position

4 Bits = 1 Nibble

Bit

0 0 l l 0 l 0 l l l l 0 0 1 0 02 03456789101112131415 1

8 Bits = 1 Byte

16 Bits = 1 Word

Information can be represented by arranging the 1s and 0s in different combinations within a 16 bit word.

Binary Concept & Binary Data Representation……… slide2

LSB

0 0 l l 0 l 0 l l l l 0 0 1 0 02 03456789101112131415 1

MSB= Most Significant Bit = Least Significant Bit

In the Next slide we will have a look ………. How a 16-bit word is used to represent the ON-OFF status of each screw terminal on a 16-point I/ O module.

Correlation of 16 bit Words to Input Signals………slide1

15 2 034567891011121314 1

Input Module

01

23

4

65

C

78

9

12

1011

1413

15

One LED for each input point. LED is ON when there is an ON signal to the corresponding input point

Correlation of 16 bit Words to Input Signals……….slide 2

Input Module

01

2

3

4

6

5

C

7

150 0 0 0 0 0 0 0 0 1 0 1 0 0 0 0

2 034567891011121314 1

+ 24 VDC

- 24 VDC

Unused bits

One LED for each input point. LED is ON when there is an ON signal to the corresponding input point

Correlation of 16 bit Words to Input Signals……….slide 3

Slot 0 Slot 1 Slot 2 Slot 3 Slot 4 Slot 5 Slot 6

0002

0806

12

04

14com

10

020406

VAC08101214

VAC

00

0911

01

0507

03

1513

0911

01

0507

03

15com

13

Power Supply CPU Input Module Output Module Input Module Output ModuleInput Module Input Module

0002

0806

12

04

14com

10

0002

0806

12

04

14com

10

0002

0806

12

04

14com

100911

01

0507

03

15com

13

0911

01

0507

03

15com

13

0911

01

0507

03

1513

020406

VAC08101214

VAC

00

0911

01

0507

03

15com

13

Input Status Table

Word 3

Word 4Word 6

Word 1

Slot 0 Slot 1 Slot 2 Slot 3 Slot 4 Slot 5 Slot 6

0002

0806

12

04

14com

10

020406

VAC08101214

VAC

00

0911

01

0507

03

1513

0911

01

0507

03

15com

13

Power Supply CPU Input Module Output Module Input Module Output ModuleInput Module Input Module

0002

0806

12

04

14com

10

0002

0806

12

04

14com

10

0002

0806

12

04

14com

100911

01

0507

03

15com

13

0911

01

0507

03

15com

13

0911

01

0507

03

1513

020406

VAC08101214

VAC

00

0911

01

0507

03

15com

13

Output Status Table

Word 5

Word 2

Correlation of 16 bit Words to Output Signals……….slide 4

PLC Memory and How it is used?………………….slide 1

Input module

Power supply

Memory

CPUOutput module

M

S

Personal Computer

PLC Memory.

We are already familiar with the basic Block diagram of Architecture of PLC.

PLC Memory and How it is used?………………….slide 2

System Memory Application Memory

Data Memory

User Memory

User Programs

Data Files

The Executive

“The Personality”

[ ROM ]

[Non- Volatile]

[ RAM ]

Volatile

PLC Memory

Application Memory……………………. Slide 1

File 0 Output Status File

File 1 Input Status File

File 2 PLC Status Bits

File 3 Internal Bits

File 4 Timer

File 5 Counter

File 6 Control

File 7 Integers

File 8 Floating Point Data

File 9 Common Interface

File 10 - 255 User Configurable

1] DATA Memory

DATA Files

Application Memory……………………. Slide 2

2] User Memory

File 0 System Program File

File 1 Reserved File

File 2 Main Ladder Program File (LAD2)

File 3 User Error Fault Routine

File 4 High Speed Counter Interrupt File

File 5 Selectable Timed Interrupt

File 6 - 15 Subroutine Ladder Program

This is the File where we will do Programming in LD Language………( Ladder Diagram)

Output Modules: Classification…………..

Discrete Output modules Analog Output Modules

Solid state OutputRelay Output

AC O/P Modules DC O/P Modules

•12, 24, 48 VAC

•120 VAC

•230 VAC

•TTL Level

•12, 24, 48 VDC

•120 VDC

•230 VDC

• 24 VDC sink

• 24 VDC source

•Relay contact

•Isolated Relay contact

•Relay contact Output

LOAD

Output Module

0

1

2

3

110 VAC

User supplied power for field devices

COMSignal from CPU Operates switch

Basic wiring for 120 VAC discrete output module

S/G Flow from CPU through an output module to the field device

CPU solves ladder

program

Output status table

Chassis backplane

Module logic circuits

Optical Isolation

Module Screw

Terminal

Field Device

Block Diagram of a Typical Output module

LatchLogic

CircuitryOptical

isolation

Triac switching circuitry

Filter

FUSE

Controlleddevice

LED

Simplified Single point Relay Output

LOAD

ON or OFF signalfrom output status table

Output Module

L-1 L-2

Relay Switching Device

Common to other output points

Sinking Output module Interface with Field Device.

Sinking Output Module

VDCOut 0Out 1Out 2Out 3Out 4Out 5Out 6Out 7

DC Com

+ DC

DC Common

Sourcing Output module Interface with Field Device.

Sourcing Output Module

VDCOut 0Out 1Out 2Out 3Out 4Out 5Out 6Out 7

DC Com

+ DC

DC Common