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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
Figure 2-5:I/O Module addressing
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
Figure 2-5:I/O Module addressing
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
Figure 2-5:I/O Module addressing
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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
Construction of a Typical I/O Module...
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THE I/O SECTION
Construction of a Typical I/O Module...
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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DISCRETE I/O MODULES
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)
DISCRETE I/O MODULES
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
DISCRETE I/O MODULES
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
DISCRETE I/O MODULES
Figure 2-10: AC Output Module SystemBlock Diagram
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DISCRETE I/O MODULES
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
DISCRETE I/O MODULES
Figure 2-12:Typical output module wiringconnection
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DISCRETE I/O MODULES
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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DISCRETE I/O MODULES
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
DISCRETE I/O MODULES
Figure 2-13:Interposing relay connection
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DISCRETE I/O MODULES
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 I/O MODULES
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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DISCRETE I/O MODULES
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
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DISCRETE I/O MODULES
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
+
-
DISCRETE I/O MODULES
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
DISCRETE I/O MODULES
Figure 2-15: Sinking and SourcingOutput devices
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Sinking device with sourcing output
module circuit
Sourcing device with sinking outputmodule circuit