Chapter 7 Input Modules

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.

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 output Analog input and output Specialty I/O modules Communication 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 DC 4 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: Status indicators Input Output Combination input or output Status indicators Screw terminals for connecting field devices Removable terminal blocks

SLC 500 Combination I/O Module 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.

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 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.

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 signals 0 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

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

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 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.

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 Image courtesy of Allen-Bradley, a Rockwell Automation business

Differential Analog Input Connections Image courtesy of Allen-Bradley, a Rockwell Automation business