2 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.
3 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.
4 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.
5 Types of I/O Modules Discrete input and output Analog input and output Specialty I/O modulesCommunication modules
6 Discrete versus Analog Inputs Discrete or digital input signals are two-state signals:Input ON or OFF, 1 or 0Analog input signals are values:0 to 10 volts DC-10 to +10 volts DC4 to 20 milliamps
7 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.
8 SLC 500 Four-Slot Chassis With I/O Module Image courtesy of Allen-Bradley, a Rockwell Automation business
9 Module Features Identifier as to type of module: Status indicators InputOutputCombination input or outputStatus indicatorsScrew terminals for connecting field devicesRemovable terminal blocks
10 SLC 500 Combination I/O Module Image courtesy of Allen-Bradley, a Rockwell Automation business
11 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.
17 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.
18 Sinking and SourcingSinking 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.
27 Basic PrinciplesNPN 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.
31 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.
32 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.
33 Bleeder Resistor Installation in an Input Circuit
34 Calculation Example If: Maximum off-state current equal to 1 milliampInput point impedance of 1 K ohmSensor leakage current equals 1.7 milliampsA 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.
35 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.
37 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.
39 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.
41 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.
42 Analog InputsTypical 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.
43 Typical Analog Inputs Analog input signals 0 to 10 volts DC 0 to 5 or 1 to 5 volts DC4 to 20 milliamps0 to 20 milliamps-20 to +20 milliamps
44 Analog Module Configurations Analog input information comes in as an input channel.Analog modules areAll inputsAll outputsCombination of input and output channels
45 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 into PLC memory.Analog data format determined by module and module configuration.
46 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.
47 Selected Allen-Bradley SLC 500 Analog Modules Image courtesy of Allen-Bradley, a Rockwell Automation business
48 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.
49 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.
50 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.