chapter 07 input modules
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Chapter 7
Input Modules
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Objectives (1 of 2)
• 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 (2 of 2)
• 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.
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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 two-state 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
• The processor always goes in left-most chassis slot.
• SLC 500 slot numbering starts with the left-most 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
Image courtesy of Allen-Bradley, a Rockwell Automation business
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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 (1 of 2)
Image courtesy of Allen-Bradley, a Rockwell Automation business
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SLC 500 Combination I/O Module (2 of 2)
• 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.
– It is most common with two-wire solid-state input devices.
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Bleeder Resistor and Leakage Current
• The bleeder resistor creates a parallel circuit so excess current can be shunted away from the 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 1K-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 addressed 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, and 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 is 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
Image courtesy of Allen-Bradley, a Rockwell Automation business
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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 is 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
Image courtesy of Allen-Bradley, a Rockwell Automation business
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Differential Analog Input Connections
Image courtesy of Allen-Bradley, a Rockwell Automation business