copyright © 2002 delmar thomson learning chapter 7 input modules

Copyright © 2002 Delmar Thomson Learning

Chapter 7

Input Modules


ObjectivesExplain 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.


Objectives (cont’d.)

Define module and sensor specifications from data sheets.

List the advantages of three-wire solid-state sensors and the disadvantages of using two-wire sensors.

Explain the differences between analog and discrete inputs.


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.


Types of I/O Modules

Discrete input and outputAnalog input and outputSpecialty I/O modulesCommunication modules


Discrete versus Analog Inputs

Discrete or digital input signals are two-state signals:Input ON or OFF, 1 or 0

Analog input signals are values:0 to 10 volts DC-10 to +10 volts DC4 to 20 milliamps


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.


SLC 500 Four-Slot Chassis With I/O Module

Image courtesy of Allen-Bradley, a Rockwell Automation business


Module Features

Identifier as to type of module:InputOutputCombination input or output

Status indicatorsScrew terminals for connecting

field devicesRemovable terminal blocks


SLC 500 Combination I/O Module



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.


Typical Wiring of Input Signals


Block Diagram of a Typical AC Input Circuit


Input Pulse with Valid Signal Areas Defined


Optical Isolator


16-Point Input Module with 8 Points Per Common


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.


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.


In Which Direction Does Current Flow?


PLC Current Flow

Many PLC manufacturers follow the theory that current flows from positive to negative.


Switch Is Source of Current: the Light Sinks to Ground


Light Is Source of Current: the Switch Sinks to Ground


Sourcing DC Input Module with a Sinking Switch


Sinking DC Input Module with a Sourcing Switch


Sourcing Two-Wire Inductive Proximity Sensor


Sinking Two-Wire Inductive Proximity Sensor


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.


Simplified Circuitry for a DC Input Module


DC Input Modules

Except for the bridge rectifier circuit, the DC input module is very similar to the AC input module.


Simplified DC Input Module Circuitry


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.


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.


Bleeder Resistor Installation in an Input

Circuit


Calculation ExampleIf:

Maximum off-state current equal to 1 milliamp

Input point impedance of 1 K ohmSensor 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.


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.


Diagram of Connection for Three-Wire Sensor


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.


Input Status Table


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.


32-Point Input Module


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.


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.


Typical Analog Inputs

Analog input signals0 to 10 volts DC-10 to +10 volts DC0 to 5 or 1 to 5 volts DC4 to 20 milliamps0 to 20 milliamps-20 to +20 milliamps


Analog Module Configurations

Analog input information comes in as an input channel.

Analog modules areAll inputsAll outputsCombination of input and output

channels


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.


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.


Selected Allen-Bradley SLC 500 Analog Modules



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.


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.


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.


1746-NI8 Input Status Table Words


SLC 500 Analog Voltage Input Specifications


Percentage of Analog Potentiometer


Single-Ended Analog Input Connections from Field

Devices



Differential Analog Input Connections


copyright © 2002 delmar thomson learning chapter 7 input modules

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