Chapter 7
Input Modules
Copyright © 2002 Delmar Thomson Learning
Objectives
 Explain the differences between positive and
negative logic and sinking and sourcing.
 Describe the available types of input modules.
 Explain the correlation between positive and
negative logic and sinking and sourcing.
 Explain how specialty I/O modules enhance a
PLC’s functionality.
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Objectives (cont’d.)
Define module and sensor
specifications from data sheets.
List the advantages of three-wire solidstate sensors and the disadvantages of
using two-wire sensors.
Explain the differences between analog
and discrete inputs.
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PLC Connection to
the Outside World
The input section of a PLC is the
physical connection between the
outside world and the CPU.
Modular PLCs use interchangeable I/O
modules.
Modules are inserted into slots in the
SLC 500 chassis.
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Types of I/O Modules
Discrete input and output
Analog input and output
Specialty I/O modules
Communication modules
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Discrete versus Analog Inputs
Discrete or digital input signals are twostate signals:
Input ON or OFF, 1 or 0
Analog input signals are values:
0 to 10 volts DC
-10 to +10 volts DC
4 to 20 milliamps
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I/O Module Positioning in Chassis
Processor always goes in leftmost
chassis slot.
SLC 500 slot numbering starts with the
leftmost slot as slot 0.
I/O modules are inserted starting at
slot 1.
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SLC 500 Four-Slot
Chassis With I/O Module
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Image courtesy of Allen-Bradley, a Rockwell Automation business
Module Features
Identifier as to type of module:
Input
Output
Combination input or output
Status indicators
Screw terminals for connecting field
devices
Removable terminal blocks
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SLC 500 Combination I/O Module
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Image courtesy of Allen-Bradley, a Rockwell Automation business
SLC 500 Combination
I/O Module (cont’d.)
Each screw terminal will have a unique
identifier called an address.
Each field device input screw terminal will
correlate to a status indicator.
Status indicator will be on when the input
point sees an input signal.
Status indicator will have same address
identifier as input screw terminal.
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Typical Wiring of Input Signals
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Block Diagram of a
Typical AC Input Circuit
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Input Pulse with
Valid Signal Areas Defined
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Optical Isolator
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16-Point Input Module
with 8 Points Per Common
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Solving Leakage Current
Problems
Carefully match input device to input
module.
Check manufacturer’s specifications.
When using a two-wire device, install
bleeder resistor when necessary.
Use a three-wire input device.
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Sinking and Sourcing
Sinking and sourcing are terms used to
describe current flow through a field
device in relation to the power supply
and the associated I/O point.
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In Which Direction
Does Current Flow?
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PLC Current Flow
Many PLC manufacturers follow the
theory that current flows from positive to
negative.
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Switch Is Source of Current:
the Light Sinks to Ground
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Light Is Source of Current:
the Switch Sinks to Ground
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Sourcing DC Input Module
with a Sinking Switch
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Sinking DC Input Module
with a Sourcing Switch
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Sourcing Two-Wire
Inductive Proximity Sensor
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Sinking Two-Wire
Inductive Proximity Sensor
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Basic Principles
NPN transistors are open-collector,
current sinking devices, which interface
to a sourcing input module.
PNP transistors are open-collector,
current sourcing devices, which
interface to a sinking input module.
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Simplified Circuitry
for a DC Input Module
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DC Input Modules
Except for the bridge rectifier circuit, the
DC input module is very similar to the
AC input module.
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Simplified DC
Input Module Circuitry
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Leakage Current in DC Modules
Maximum off-state current:
The maximum amount of leakage current
allowed in an input circuit that will keep the
input circuit in an off state.
Too much current leaking from input
device will be seen as a valid on signal by
input module all the time.
Most common with 2-wire solid state input
devices.
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Bleeder Resistor
and Leakage Current
Bleeder resistor creates a parallel circuit
so excess current can be shunted away
from input point.
Refer to manufacturers’ specifications
for calculations.
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Bleeder Resistor
Installation in an Input Circuit
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Calculation Example
If:
Maximum off-state current equal to 1 milliamp
Input point impedance of 1 K ohm
Sensor leakage current equals 1.7 milliamps
A 1-K ohm resistor in parallel with the input
will split current going into module input point
in half to .85 milliamps.
Other .85 milliamps go through resistor to
ground.
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Solving Leakage
Current Problems
Check manufacturer’s specifications
and carefully match input device to input
module.
When using a two-wire device, install
bleeder resistor when necessary.
Use a three-wire input device.
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Diagram of Connection
for Three-Wire Sensor
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Discrete Input
Status Table Word
16-point discrete input modules are
assigned one word in the input status
table.
A 16-point discrete input module
residing in slot 3 would be addresses as
I:3.0, bits 0 through 15.
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Input Status Table
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Discrete I/O Module
With 32 Points
Because input status word is 16 bits wide,
there are 16 bits available for up to 16 I/O
screw terminals.
A 32-point I/O module will require two 16-bit
words to accommodate all I/O points.
For a 32-point input module in slot 3, two
words, I:3.0 and I:3.1 will be created as part
of the I/O configuration.
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32-Point Input Module
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Input Addressing for Upper 16
Bits
 Screw terminal 16 address would be I:3.1/0.
 Screw terminal 17 address would be I:3.1/1.
 Screw terminal 18 address would be I:3.1/2.
 Screw terminal 31 address would be I:3.1/15.
Can be entered on ladder as I:3/31, will display as
set up in properties.
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Analog Inputs
Typical analog signals come from
temperature, pressure, position, motor
speed.
Analog input modules convert analog
signals to digital words.
Analog input signals are current or
voltage.
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Typical Analog Inputs
Analog input signals
0 to 10 volts DC
-10 to +10 volts DC
0 to 5 or 1 to 5 volts DC
4 to 20 milliamps
0 to 20 milliamps
-20 to +20 milliamps
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Analog Module Configurations
Analog input information comes in as an
input channel.
Analog modules are
All inputs
All outputs
Combination of input and output channels
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Analog Input Signals
Represent Values
A potentiometer supplying a 0 to 10 V
DC input signal to an analog input
module could send a converted signal
of 0 to 32767 into PLC memory.
Analog data format determined by
module and module configuration.
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Local Versus Remote I/O
Local analog modules in a SLC 500
system automatically send the
converted signal into the input status
table.
Remote I/O analog signals require block
transfers.
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Selected Allen-Bradley
SLC 500 Analog Modules
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Image courtesy of Allen-Bradley, a Rockwell Automation business
Analog Input Modules and the Input
Status Table
Each analog channel will be
represented by a entire word in the
input status table.
A two-channel analog input module will
have two words in the input status table
to represent the binary representation of
the analog voltage or current.
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Analog Data Is Represented
as Whole Word Data
Analog signals are not digital signals
represented as 1s and 0s.
Analog information is represented as a
16-bit signed integer in the data table.
Each channel will require one word.
Data table will reserve one word for each
channel.
This part of the I/O configuration process.
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Input Word Assignment
1746-NI8 is an 8-channel analog input
module.
Eight words will be assigned to the input
status table as part of the I/O
configuration.
Module in slot 4 will have words I:4.0
through I:4.7, one for each channel.
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1746-NI8
Input Status Table Words
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SLC 500 Analog Voltage
Input Specifications
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Percentage of
Analog Potentiometer
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Single-Ended Analog Input
Connections from Field Devices
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Image courtesy of Allen-Bradley, a Rockwell Automation business
Differential Analog
Input Connections
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Image courtesy of Allen-Bradley, a Rockwell Automation business