PLC Inputs and Outputs 240

This class covers different types, configurations, capacities, and current conversions for PLC I/Os. Includes an Interactive Lab.

Class Details

Class Name:
PLC Inputs and Outputs 240
Number of Lessons:
Additional Language:

Class Outline

  • Objectives
  • The PLC Input/Output Section
  • I/O Screw Terminals
  • I/O Configuration
  • I/O Power Conversion
  • Discrete I/Os
  • Data I/Os
  • Sinking I/Os
  • Sourcing I/Os
  • Noise Filtering and Surge Suppression
  • Troubleshooting the Input/Output Module
  • Troubleshooting I/O Devices
  • Forcing and Disabling I/Os
  • Summary


  • Describe the PLC input/output section.
  • Explain the function of I/O screw terminals on a PLC I/O module.
  • Distinguish between fixed I/O and expandable I/O configurations.
  • Explain how I/O power conversion works.
  • Describe discrete I/Os.
  • Describe data I/Os.
  • Describe sinking I/Os on a DC circuit.
  • Describe sourcing I/Os on a DC circuit.
  • Describe noise filtering and surge suppression.
  • Explain how to troubleshoot an I/O module.
  • Explain how to troubleshoot I/O devices.
  • Explain how to use the force command and the disable command.



Vocabulary Term Definition
address The unique location assigned to each I/O module so that it may be correctly referenced and signaled by the PLC program.
analog A continuously variable signal. Analog signals differ from digital signals in that small fluctuations in the analog signal are meaningful.
analog to digital converter A device that changes the continuous fluctuations in voltage from an analog device into a digital signal.
backplane The area on the back of a PLC enclosure where various logic and control elements are interconnected. The backplane is usually a printed circuit board that is connected to other printed circuit boards.
binary A number system used for PLCs that has either 0 or 1 as a value. Using binary, a function is either on or off.
binary coded decimal A binary number system in which each decimal digit from 0 to 9 is represented by four binary digits (bits). The four positions have a weighted value of 1, 2, 4, and 8, ranging from right to left.
bridge rectifier A configuration of semiconductor devices that changes AC to full-wave pulsating DC.
channel The input/output of an analog module. Channels can be single ended or differential pairs.
chassis The frame or enclosure of a modular PLC.
common The screw terminal on a PLC that connects to the power source.
conversion electronics The internal electronic process in a PLC that converts AC to DC, and DC to AC.
data I/O A PLC input/output that processes many different types of signals over a wide range.
debug To test a PLC for errors.
decimal A numbering system that uses ten digits, from 0 to 9, arranged in a series of columns to represent all numerical quantities. Each column or place value has a weighted value of 1, 10, 100, 1000, and so on, ranging from right to left.
differential pair A type of analog channel in which each input has a connection to its own input terminal and corresponding terminal.
digital Consisting of information that is input or output electronically as a series of pulses or signals often resulting in binary strings of 0s and 1s.
digital multimeter A device that can measure voltage, current, or resistance. A digital multimeter is the most versatile and common meter used today for electrical maintenance.
digital to analog converter A device for converting digital signals into continuous analog signals. The converter usually buffers the input so that the output remains the same until the input changes. A typical converter provides two analog output channels ranging from 0 to +10 volts with eight bits per channel resolution; also provides two logic level outputs for external device control.
disable command A command used to prevent an output device from operating. Disable is the opposite of the force command, and it can be used to prevent the operation of one or all of the output devices.
discrete A signal that has two states, ON and OFF.
discrete I/O An input/output device or module that recognizes only binary on/off signals. A pushbutton is an example of a discrete input device, and a light is an example of a discrete output device.
electrical noise An AC power line disturbance caused by sudden changes in the load. Electrical noise is problematic to solid state devices because they cannot differentiate between an intended electrical pulse and an unintended electrical spike.
expandable I/O An input/output section that can be expanded to allow more inputs and outputs.
expansion module An I/O unit that is separate from the controller of the PLC.
field input/output device A peripheral device that provides input to or sends output from a PLC.
fixed I/O I/Os located onboard the PLC in a unit that also contains the processor, input/output modules, and power supply. With fixed I/Os, you cannot add more inputs or outputs than the number that have already been allotted.
force I/O command A troubleshooting command used to override the input/output status of an application in order to test or debug a PLC program. The force command overrides any safety features in the PLC program.
force I/O lamp An indicator lamp on a PLC that confirms inputs and outputs are in forced operating mode.
I/O module The section of an expandable PLC to which input and output devices are connected.
I/O section The section of a PLC to which input and output devices are connected. The I/O section also converts power levels.
inductive device An AC electrical load in which the voltage wave reaches its peak before the current.
input device A device, usually a type of sensor, that sends information into a PLC. Inputs are connected to the PLC by input relays.
input section The jack used to physically connect an input device to a PLC. The input relays transfer signals to the internal relays.
input/output section The section of a PLC to which input and output devices are connected. The input/output section also converts power levels.
local I/O An I/O that is in the same rack as the processor.
multiplexing A type of wiring that allows multiple signals to be processed through a single channel. Multiplexing is used to save on wiring and I/O ports.
output device A device that performs a mechanical action after receiving the electrical signal to do so from the PLC output relays.
output section The jack used to physically connect an output device to a PLC. The output modules transfer signals from the internal relays.
printed circuit board A layered construction of material used to mechanically support and electrically connect electronic components using conductive pathways, or traces, etched from copper sheets laminated onto a non-conductive surface.
program mode An operational mode of the CPU of a PLC that is used for monitoring and changing the PLC program.
programming device A device, usually a personal computer, used to enter instructions into the PLC. Some programming devices are small enough to fit in your hand.
rack An enclosure with slots in it that is to connect multiple parts of a PLC.
remote I/O An Input/output module that is located far away from the CPU of the PLC.
shielded cable Cable that has an outer layer of insulation covering an innerlayer of conducting material. Shielded cable is used to reduce electronic noise and voltage spikes.
single-ended A type of analog channel in which all the input commons are tied together.
sink/source I/O module An I/O module with circuitry that allows current to flow in either direction.
sinking A type of circuit in which the I/O device provides current to the I/O module.
snubber circuit An electrical circuit used to suppress electrical spikes. Snubbers are often used with inductive loads.
sourcing A type of circuit in which the I/O module provides current to the I/O device.
status lamp A light-emitting diodes on a PLC that provides information on the operating condition of PLC I/Os.
terminal block A device that joins wires or cables. Terminal blocks typically snap into a metal rail or are screw mounted on the panel of a control enclosure.
test mode An operational mode of the CPU of a PLC that checks if inputs and outputs are working without actually energizing the input/output circuits or devices.
vibrating voltage tester A testing device that vibrates when a current is detected. Also called a "wiggy", it is small and durable, but may be replaced by more versatile and safer DMMs.
voltage spike A sudden, short surge in voltage. Voltage spikes can be caused by lightning, power outages, short circuits, or power transitions in large equipment on the same power line.