CH02 PLC Hardware Components

7/30/2019 CH02 PLC Hardware Components

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1D.Wilcher

CHAPTER 2A: PLC HARDWARE

COMPONENTS


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CHAPTER OBJECTIVES

1. List and describe the function of thehardware components used in PLC systems.

2. Describe the basic circuitry andapplications for discrete and analog I/Omodules, and interpret typical I/O and CPUspecifications.3. Explain I/O addressing.4. Describe the general classes and typesof PLC memory devices.

5. List and describe the different types ofPLC peripheral support device available.

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THE I/O SECTION

1.The I/O (Input & Output) interfacemodules provides the equivalents of

eyes,ears, and tongue to the brain of aPLC.2.The I/O section consists of an I/O rackand individual I/O modules. (See Figure 2-1)3. The I/O system provides an interfacebetween the hardwired components in the

field and the CPU.4. The input interface allows statusinformation regarding processes to becommunicated to the CPU.

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0 1 2 3 4 5 6

Power

supply

Processormodule

Slot

I/O Modules

Figure 2-1:Typical I/O Section

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THE I/O SECTION

1.The chassis is a physical hardwareassembly that houses devices such as I/O

modules, processor modules, and powersupplies.2. Chassis come in different sizesaccording to the number of slots they

contain.3. In general, they can have 4, 8, 12, or 16cells.

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1.A logical rack is an addressable unit

consisting of 128 input points and 128output points. (See Figure 2-2).2. A rack uses 8 words in the input imagetable file and 8 words in the output imagetable. (See Figure 2-3)3. A word in the output image and itscorresponding word in the input image table

file are called an I/O group.4. A rack can contain a maximum of 8 I/Ogroups (numbered from 0 -7) for up to 128discrete I/O.

THE I/O SECTION

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0 1 2 4 53 6 7

Chassis

Processor128 I/O rack

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Figure 2-2:Typical Logical Rack


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O:00 0 1 2 3 4 5 6 7 10 11 12 13 14 15 16 17

O:01 0 1 2 3 4 5 6 7 10 11 12 13 14 15 16 17

O:02 0 1 2 3 4 5 6 7 10 11 12 13 14 15 16 17O:03 0 1 2 3 4 5 6 7 10 11 12 13 14 15 16 17

O:04 0 1 2 3 4 5 6 7 10 11 12 13 14 15 16 17

O:05 0 1 2 3 4 5 6 7 10 11 12 13 14 15 16 17

O:06 0 1 2 3 4 5 6 7 10 11 12 13 14 15 16 17

O:07 0 1 2 3 4 5 6 7 10 11 12 13 14 15 16 17

7 10

Output image table

Words

I:00 0 1 2 3 4 5 6 7 10 11 12 13 14 15 16 17

I:01 0 1 2 3 4 5 6 7 10 11 12 13 14 15 16 17

I:02 0 1 2 3 4 5 6 7 10 11 12 13 14 15 16 17

I:03 0 1 2 3 4 5 6 7 10 11 12 13 14 15 16 17I:04 0 1 2 3 4 5 6 7 10 11 12 13 14 15 16 17

I:05 0 1 2 3 4 5 6 7 10 11 12 13 14 15 16 17

I:06 0 1 2 3 4 5 6 7 10 11 12 13 14 15 16 17

I:07 0 1 2 3 4 5 6 7 10 11 12 13 14 15 16 17

Input image table

Words

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Figure 2-3:Typical I/O Image Tables


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THE I/O SECTION

1.One benefit of a PLC system is the

ability to locate the I/O modules near fielddevices. (minimize the amount of wiringrequired)2. Remote Rack located away from the

processor module (See Figure 2-4)3. To communicate with a remote rack, aspecial communications network is created.

4. Each remote rack requires a uniquestation number to distinguish one fromanother.

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PLC

P

ower

Pro

cessor

Input

O

utput

Com

munication

Remote I/O

Power

Input

Output

Communication

Drum

Hopper

Sensor

ON/OFF

control

MotorPushbutton

Programming Unit

Figure 2-4:Remote I/O Rack

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THE I/O SECTION

1.The remote racks are linked to the local rackthrough a communications module.

2.Cables connect the modules with each other.3.If fiber optic cable is used between the CPUand I/O rack, its possible to operate I/Opoints from distances greater than 20 miles

with no voltage drop(s).4. Fiber optic cable will not pick up noisecaused by adjacent high power lines orequipment normally found in an industrial

environment. (Coaxial cable is more susceptibleto this type of noise.)5. Coaxial cable will allow remote I/O to beinstalled at distances greater than 2 miles.

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THE I/O SECTION

1.The location of a module within a rack andterminal number of a module to which an inputor output device is connected will determine

the devices address (See Figure 2-5).2.Each input and output device must have aspecific address.3.The processor uses this information to

identify where the device is located to monitoror control.4.Field wiring is easily connected to the I/Omodule housing as well as disconnecting to the

PLC.5. Indicator LEDs are added to the module fora quick ON or OFF status of each I/O circuitactivated. Blown fuse indicators are available

on some I/O modules.D.Wilcher


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0 1 2

Processor

Discretem

odule

1

2

3

4

0

5

6

7

1

2

3

4

0

5

6

7

I 0

5

0 0

I 0 0

O 0

6

0 0

O 0 0

Inputs OutputsType Slot Word Bit Type Slot Word Bit

Address Address

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Figure 2-5:I/O Module addressing


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O 0 : 0 1 / 00

Terminal number (bit address)

Bit delimiter

Group (0-7): 8 groups in a rack

Rack no.

File no.

File type ( O - output: I - input)

File type


AB (Allen Bradley) PLC 5 addressing format

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Memory

address

Real-world

address

I 1 : 3 . 0 / 01

File type

File number

Element number

Subelement number

Bit number

Terminal number

For terminals above #15

Slot number

Module type


AB (Allen Bradley) SLC 500 addressing format

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1

0

1

0I 2 0

Notu

sed O 2 0

Inputs OutputsType Slot Word Bit Type Slot Word Bit

Address Address

0 1 2

Processor

Analog

module

Notu

sed

LS_3 pump_14

I :3 O : 4

3 14


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THE I/O SECTION

Addressing Elements

1.Type determines if an input or output is

being addressed.2.Slot The physical location of the I/Omodule. NOTE: Maybe a combination of therack number and the slot number when using

expansion racks.3. Word and Bit- Used to identify the actualterminal connection in a particular I/O module.

a) A discrete module usually uses only 1 word.

b) The design of a PLC determines whetherthe system is a capable of being addressedflexibly or its rigid in its addressing method.

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THE I/O SECTION

What is a Flexible System?

1.Individual slot and point addresses are

normally determined by the sequences in whichthe I/O racks are connected together.

2. In the case of some small PLCs, the system

contains 1 rack and therefore has I/Oaddressing fixed by the manufacturer.

3.Actual address labeling varies greatly from

manufacturer to manufacturer.

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THE I/O SECTION

Construction of a Typical I/O Module

1.Consists of a printed circuit board (PCB) and

terminal assembly. (See Figure 2-6)

2.The PCB contains the electronic circuitryused to interface the circuit of the processor

with that of the input or output device .

3. Its designed to plug into a slot or connectorin the I/O rack or directly into the processor.

4. The terminal assembly is attached to thefront edge of the PCB and is used for makingfield-wiring connections.

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THE I/O SECTION

Construction of a Typical I/O Module...

1.The module contains terminals for each I/O

connection and to the power supply used topower the I/O.

2. Most modules have plug-in wiring terminal

strips.

3. If there is a problem with a module/terminal strip, the entire unit is removed and

replaced with a new one.

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6

5

7

4

3

2

1

0

6

5

7

4

3

2

1

0

Input Output

Status

Input Output

Output

connections

Input

connections

Statusindicators

THE I/O SECTION

Figure 2-6: Construction of a TypicalI/O Module

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1.I/O module cards can be placed anywhere inthe rack but are normally grouped together for

ease of wiring.

2.I/O modules can be 8, 16,or 32 point cards .

3.The number refers to the number of I/Omodules available.

4. The standard I/O module has 8 inputs or

outputs.

5.A high-density module may have up to 32I/O. Advantage: Possible to install 32 I/O in 1

slot for space savings.

THE I/O SECTION


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THE I/O SECTION


1.High density module disadvantage is notbeing able to handle much current per output.

2. High density modules 32 point cards usually

have at least 4 common points .

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DISCRETE I/O MODULES1.The most common type of I/O interfacemodule is the discrete type.

2.This type of interface connects field inputdevices of the ON/OFF nature. (Ex. Selectorswitches, pushbuttons, and limit switches)

3. Output control is limited to devices such aslights, small motor, solenoids, and motorstarters that require simpleON/OFFswitching.

4. The classification of discrete I/O coversbit oriented inputs and outputs.

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DISCRETE I/O MODULES

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1.Each discrete I/O module is powered bysome field-supply voltage source.

2.Since these voltages can be of differentmagnitude or type, I/O modules are availableat various ac and dc voltage ratings (See Table2-1).

3. They receive their module voltage andcurrent for proper operation from thebackplane of the rack enclosure into which

they are inserted.

4. Power from this supply is used to power theelectronics, both active and passive, that

reside on the I/O module PCB.


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TABLE 2-1

COMMON RATINGS FOR DISCRETE

I/O INTERFACE MODULES

Input Interfaces Output Interfaces

12V ac/dc/24 Vac/dc 12 - 48 V ac

48V ac/dc 120V ac

120V ac/dc 230V ac

230V ac/dc 120V ac

5V dc (TTL level) 230V dc

5V dc (TTL level)

24V dc

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DISCRETE I/O MODULES1.The relative higher currents required by theloads of an output module are supplied by user-supplied power.

2.Module power supplies may be rated for 3A,4A, 12A, or 16A depending on the type andnumber of modules used.

3. An AC discrete Input Module is built usingboth Analog Power and Digital Electronics. (SeeFigure 2-7)

4. Power from this supply is used to power theelectronics, both active and passive, thatreside on the I/O module PCB.

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Bridge

rectifier

Zener

diode

level

detection

Isolator Logic

Input statusindicator

To

processor

(5Vdc)

Input

signal(120Vac)

L1

L2

Power Electronics Digital Electronics

Logic circuits Backplane

LED

Opto-electrical

isolation

Input

conditioning

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Figure 2-7:AC Input Module SystemBlock Diagram


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L

E

D

Optical

Isolator

(120Vac)

L2

L1

PBR1

R2

Bridge

Rectifier

ZP

R3 D1

To logic

circuitry

(5Vdc)

(120Vac)


Figure 2-8:Simplified SubcircuitSchematic Diagram of an AC Input

Module

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L1

1

2

3

L2

120Vac

L1 L2

Input from

field

device

Input status

indicator

Input module

terminal board


Figure 2-9: Typical input module wiringconnection

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(120Vac)

Power Electronics

Electronic

switchIsolatorLogic

Outputstatus

indicator

From

processor(5Vdc)

L1

L2

Lamp

output

Digital Electronics

OutputLogic circuitsBackplane

LED

Opto-electricalisolation

Logic circuits Output drivers


Figure 2-10: AC Output Module SystemBlock Diagram

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Figure 2-11: AC Output Module DriverSubcircuit Diagram

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L1

1

2

3

L2

120Vac

L1 L2

Input status

indicator

Output module

terminal board

4

Output field

indicator

Module

blown fuse

indicator


Figure 2-12:Typical output module wiringconnection

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AC output module

1.Individual ac outputs are usually limited bythe size of the triac to 1A or 2A.

2.The maximum current load for any 1 moduleis also specified.

3. To protect the output module circuits,specified current ratings should not beexceeded.

4. For controlling larger loads such as largemotors, a standard control relay is connectedto the output module.

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AC output module

1.The contacts of the relay can then be usedto control larger load or motor starter.

2.When a control relay is used in this manner,

it is called an interposing relay. (See Figure 2-13)

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CR

Motor

M

Motor

starter

coil

CR

Interposing

relay coil

OL

L1 L2 L3

MM M

T1 T2 T3


Figure 2-13:Interposing relay connection

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Discrete output module1.Discrete output modules are used to turnreal world output devices either ON or OFF.

2.These modules can be used to control any 2-state device: Available in ac or dc versions.

3. They are made in various voltage ranges andcurrent ratings.

4.Output modules can be purchased withtransistor, triac, or relay devices.

5. Triac outputs can be used only for controlof ac devices.

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Discrete output module1.Transistor outputs can be used for controlof dc devices.

2.Relay outputs can be used with ac or dcdevices.

3.Switching response of relays are slower thansolid state outputs (triac or transistor)because of the mechanical resistance of thespring used to return the contacts to theiroriginal position.

4.Allen Bradley (AB) modules are color codedfor each identification (See Table 2-2).

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Red . . . . . . . . . . . ac inputs/outputs

Blue . . . . . . . . . . . dc inputs/outputsOrange . . . . . . . . . Relay outputs

Green . . . . . . . . . . Specialty modules

Color Type of I/O

Table 2-2:

AB Module Color Code Identification

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DC Field Devices1.The design of dc field devices typicallyrequires a specific sinkingor sourcingcircuit.

2.Depends on the internal circuits of thedevice.

3.Sinking and sourcing references are terms

used to describe a current flow relationshipbetween field input and output devices in acontrol system.

4.Sourcing I/O circuits (source) current tosinking field devices.

5. Sinking I/O circuits (receive) sink current

from sourcing field devicesD.Wilcher


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DC Field Devices

1.Another common name used for Sinkingcircuit is Low Side driver.

2.Another common name used for Sourcingcircuit is High Side driver.

3.Sinking and sourcing references are termsused to describe a current flow relationshipbetween field input and output devices in acontrol system.

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Field device

dc power

supply

dc

input

circuit

Vdc

Input

I

+

-

Field device

dc power

supply

dc

input

circuit

dc com

Input

I

+

-


Figure 2-14: Sinking and Sourcing Inputdevices

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Sinking device with sourcing input

module circuit

Sourcing device with sinking inputmodule circuit


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Field devicedc powersupply

dcinput

circuit

dc com

Out

I+

-

Vdc

Field devicedc powersupply

dc

input

circuit

dc com

Out

I+

-

Vdc


Figure 2-15: Sinking and SourcingOutput devices

D.Wilcher

Sinking device with sourcing output

module circuit

Sourcing device with sinking outputmodule circuit

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CH02 PLC Hardware Components

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