Chapter 4: Input/Output Modules and
Installation
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Input/Output Modules
Learning objectives

Understand

Wiring of typical sensors and actuators to PLC I/O modules

Surge protection for certain types of devices

Typical control cabinet layout

Typical installation wiring
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Input/Output Modules
Overview

Input/Output (I/O) Modules provide Physical Interface
between


PLC Processor and
Field Devices:


Switches, Lamps Valves etc…
Common Characteristics of I/O Modules

Removable Terminal Blocks (Retractable)



Wiring is connected to the terminal block
T
Terminal
i l block
bl k is
i plugged
l
d into
i t the
th module
d l
Isolation

Isolates field wiring from the PLC internal circuitry


PLC power supply provides low-voltage power unlike a typical field wiring
Diagnostic Indicators


Discrete Input modules display an indicator for each input channel that is on
Discrete Output modules display an indicator for each output channel that is
on
o
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Input/Output Modules
Isolation between I/O and PLC
Isolation Barrier
L1 (Line)
L1 (Line)
(Li )
Fuse
PLC
Processor
Neutral
L1

Neutral
Power
S l
Supply
Input
Module
Output
Device
Output
Module
Neutral
I/O modules provide isolation between field wiring and PLC internal circuitry:



Ground loops
Noisy electrical signals
Fault on I/O wiring
Isolation prevents events from interfering with the PLC and causing a module failure
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Input/Output Modules
Sinking & Sourcing

What is Sinking
g and Sourcing?
g

The flow of current (power) when the I/O device is active

An Output Device is designated as Sourcing if current
flows OUT when the output device is active

An Output Device is designated as Sinking if current
flows IN when the output device is active
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Input/Output Modules
Sinking & Sourcing

What is Sinking
g and Sourcing?
g

The flow of current (power) when the I/O device is active

An Input Device is designated as Sourcing if current
flows OUT when the output device is active

An Input Device is designated as Sinking if current flows
IN when the output device is active
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Input/Output Modules
Sinking & Sourcing
A Sourcing field Sensor is usually
connected
t d to
t a PLC input
i
t why?
h ?
Sourcing
Field
Sensor
Current
Sinking
PLC
Input
Sinking
Field
Sensor
Current
Sourcing
PLC
Input
Sourcing
PLC
Output
Current
Sinking
Field
Device
Sinking
PLC
Output
Current
Sourcing
Field
D i
Device
Arrows indicate the direction of the current flow
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Input/Output Modules
Focus
 Primarily concern with sinking PLC inputs and sourcing
PLC outputs
t t since
i
th
thatt is
i th
the majority
j it off ttypes off
connections.

Note that most discrete sensors can be purchased in
either a sinking or sourcing configuration. Make sure you
specify
if th
the correctt one!!
!!
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Discrete Input Modules
A Discrete Input module senses status of
devices with only two states
 on/off,
on/off open/closed,
open/closed running/stopped

Types of discrete inputs



Switches - many kinds

Pushbutton

Li it
Limit

Level
Proximity
R l contacts
Relay
t t
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Discrete Input Modules
Standard Discrete Input Voltages
 120 v AC/DC (can
(
handle
h dl either)
ith )
 220 v AC/DC
 ≈10 - 30 v DC
 ≈10 - 30 v AC
 ≈20 - 60 v DC
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Discrete Input Modules
Discrete Input
p Module Block Diagram
g
Powered from the field
Powered from PLC
Threshold detector



Senses when input device is on
When input device voltage > minimum-on-state voltage  devised is sensed to be on
When device voltage < maximum-off state voltage  devised is sensed to be off
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Discrete Input Modules
Discrete Input Module Block Diagram
Powered from the field
Powered from PLC
Optical Isolator

Provides electrical isolation between field wiring and the PLC internal circuitry




Light-Emitting Diode & Photoelectric transistor
S it h Cl
Switch
Closed
d Current
C
t flow
fl
th
through
h LED Generates
G
t Light
Li ht  Light
Li ht T
Triggers
i
currentt fl
flow th
thru Photoelectric
Ph t l t i
transistor  Sensed by PLC digital logic
Circuitry to the LHS of isolator powered from the field
Circuitry to the RHS powered from internal PLC voltage source
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Discrete Input Modules
Discrete Input Module Block Diagram
P
Powered
d from
f
the
th field
fi ld
P
Powered
d from
f
PLC
Noise and Dist
Disturbance
rbance

Capacitors and Resistors are typically used to form simple noise filter
B id Rectifier
Bridge
R tifi

Converts AC to full-wave rectified DC
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Discrete Input Modules
AC Inputs
L1
(Line)
I/O Module
Term. Strip
L2
(Load)
Black
(120 VAC)
White
(120 VAC)
Com
Some modules need this
connection for balancing
AC Inputs

Power flows into the Input Module when the field device is on



Hence the Input Module is characterized as a Sinking Module
There is a least one common per module. Some modules have a common for every 4
input channels
Why would you need more than one common?
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Discrete Output Modules
Discrete Output Modules:




Used to turn real-world output deices either on or off
Can be used to control any two state device
Available in AC and DC versions
Available in various voltage ranges and current capabilities
Examples of Discrete Devices Controlled by PLC



Indicator Lamps, Alarm Lamps
Motor Starters,, Electric Valves,, Electric Fans
Control Relays, Heater Relay
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Discrete Output Modules
Standard Discrete Output
p Voltages
g









120 v AC
220 v AC
≈12 - 48 v AC
120 v DC
220 v DC
≈12 - 48 v DC
AC Di
Discrete
t Output
O t t Modules
M d l handle
h dl only
l Ac
A Signals
Si
l
DC Discrete Output Modules handle only DC Signals
Relay output Modules handle both Ac and DC signals
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Discrete Output Modules
Block Diagram
Optical Isolator



Provides electrical isolation between PLC and internal circuitry and field wiring
Circuitry on LHS of Optical Isolator powered by internal PLC voltage
Circuitry on RHS of Optical Isolator powered from the field
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Discrete Output Modules
AC Outputs
L1
(Line)
L2
(Load)
I/O Module
Term. Strip
M
Common
AC Signals

Output Modules that handle Ac signals are Sourcing Modules

Power flows out of the Module when the Switch (“Triac” ) is ON
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Input/Output Modules
Wi i
Wiring

Best to sketch the diagram
g
on a p
piece of p
paper
p before
wiring


Easier to visualize the whole picture
S
Suppose
you h
have many iinputs
t tto wire?
i ?

Start the sketch with one input first



If you figure out the first, the rest will follow
Next simply add second input
third input etc…
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Input/Output Modules
Si ki IInput M
Sinking
Module
d l Wi
Wiring
i

Suppose:
One Input
Power Supply
Wiring Terminals
Input 0
Input 1
Input 2
Input 3
Wire this simple circuit
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Input/Output Modules
Wi i iin a Si
Wiring
Simple
l Ci
Circuit
i
Input 0
I
Input
t1
Input 2
Input 3
Common
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Input/Output Modules
Wi i Addi
Wiring
Additional
i
l Input
I
D
Device
i
Input 0
I
Input
t1
Input 2
Input 3
Common
Wiring an additional input device
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
Connect 3-wire sourcing
g sensor

Sourcing Sensor  output is positive
-
OUT
TPUT
Input/Output Modules
Wi i three-wire
Wiring
h
i Sensor
S
+
Sourcing
g
Sensor
Input 0
Input 1
Input 2
Input 3
Common
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Input/Output Modules
Th
Three-wire
i Sensor
S
Wi
Wiring
i
Complete Wired Circuit
-
OUT
TPUT

+
Sou c g
Sourcing
Sensor
Input 0
Input 1
Input 2
Input 3
Common
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