PLC Inputs and Outputs 231

“PLC Inputs and Outputs” provides an overview of the main types of input/output modules and input/output devices, their primary functions, and their roles in a PLC process. In a PLC system, the input/output modules are connected to the input/output devices that send and receive electrical signals throughout a process. Input/output modules may operate using alternating current (AC) or direct current (DC), and may be analog or discrete, depending on the type of electrical signals they process.

Having a foundational knowledge of the functions and capabilities of the input/output modules in a PLC helps users understand basic PLC operation. Being aware of the different types of input/output modules and their capabilities is essential to working with PLC systems.

Class Details

Class Name:
PLC Inputs and Outputs 231
Version:
2.0
Difficulty:
Beginner
Number of Lessons:
13
Related 1.0 Class:
PLC Inputs and Outputs 240

Class Outline

  • Programmable Logic Controllers
  • Types of PLC
  • Input/Output Module
  • Review: I/O Modules
  • Discrete I/O
  • Analog I/O
  • PLC Process
  • Wiring Configurations for I/O Modules and Devices
  • Review: Analog and Discrete I/O
  • Common Issues with I/O Devices and I/O Modules
  • Troubleshooting I/O Devices and I/O Modules
  • Troubleshooting a PLC using Force Commands
  • Final Review

Objectives

  • Describe the basic functions of a PLC.
  • Distinguish between fixed PLCs and modular PLCs.
  • Describe an input/output module.
  • Describe discrete I/O signals and modules.
  • Describe analog I/O signals and modules.
  • Explain how a PLC processes a signal.
  • Contrast sinking and sourcing configurations.
  • Describe protection methods for power line interference caused by I/O devices and I/O modules.
  • Describe how to troubleshoot an I/O device and I/O module.
  • Describe the commands used when troubleshooting a PLC.

Job Roles

Certifications

Glossary

Vocabulary Term Definition
AC Alternating current. Electricity that reverses direction at regularly recurring intervals of time. AC switches direction 60 times per second, or 60 hertz.
actions An output instruction in a PLC program. An action is the result of an input condition being met.
address The unique identification of the location of an input device, output device, or saved data within the CPU. Addresses may be labeled on input/output modules for connected input/output devices.
alternating current AC. Electricity that reverses direction at regularly recurring intervals of time. Alternating current switches direction 60 times per second, or 60 hertz.
amperage The strength or intensity of an electrical current. Amperage is measured in amperes.
analog An electrical signal that continuously varies within a specific range. Analog signals communicate variable conditions, such as pressure or temperature.
analog to digital converter ADC. A component within an analog input module that changes variable signals to binary numbers. An analog to digital converter changes the continuous fluctuations from an analog input device to a digital, or binary, signal for the CPU.
backplane The portion of a modular PLC where I/O modules, the CPU, and power supply are connected. The backplane provides communication paths for electrical signals.
binary digits A 1 or a 0 in the binary numbering system. Binary digits are what a CPU uses to process information.
central processing unit CPU. The microprocessor device inside a computer that controls system activities. The central processing unit executes a PLC program and performs all logic activities for the system.
chassis An enclosure with slots in it that is used to connect and hold the multiple units of a modular PLC. A chassis typically allows room for additional units.
commands A specific instruction given to a computer to perform a task or function. Commands can be used to override specific instructions for a CPU.
common terminal A shared connection point on an input/output module that connects to a power source. A common terminal is at the bottom of the screw terminal.
condition An input instruction in a PLC program. A condition must be met by PLC components in order to result in an action.
CPU Central processing unit. The microprocessor device inside a computer that controls system activities. The CPU executes a PLC program and performs all logic activities for the system.
data file The area of a CPU's memory that contains the status of input devices, output devices, and other program instructions. Data files store information, such as addresses or conditions for associated devices.
DC Direct current. Electricity that forms when electrons flow in one continuous direction. DC is used in many PLC circuits.
digital multimeter DMM. A device that can measure voltage, current, or resistance. A digital multimeter is used for electrical maintenance and troubleshooting.
digital to analog converter DAC. A component within an analog output module that changes binary numbers to analog signals. A digital to analog converter changes the binary numbers from the CPU into a continuous variable signal for an analog output device.
direct current DC. Electricity that forms when electrons flow in one continuous direction. Direct current is used in many PLC circuits.
discrete An electrical signal that has only two states. Discrete signals are either on or off.
electrical noise A power line disturbance caused by sudden changes in the load. Electrical noise is problematic to a PLC because the components cannot differentiate between an intended electrical spike and an unintended electrical spike.
electrical signal A current that carries data from one device to another. Electrical signals are either analog or discrete.
electromagnetic Electrical noise in a circuit that can interrupt or degrade the performance of the circuit. Electromagnetic interference can be reduced by using shielded cable or fiber optic cables.
field device A component that connects to an input/output module on a PLC and performs an action. Each field device, or input/output device, has its own electrical requirement.
fixed PLC A type of PLC in which the central processing unit, input/output module, and power supply are housed in a self-contained unit. A fixed PLC is used for applications requiring basic process control.
force command A troubleshooting command on a CPU used to override the input/output status of a device in order to test or debug a program. The force command is activated and removed using a programming device.
I/O module Input/output module. The portion of a PLC that contains connection points for input/output devices and circuitry to convert electrical current. An I/O module may be input only, output only, or a combination.
I/O points Input/output points. A connection point on an input/output module for a field device wire. I/O points allow an electrical connection between field devices, or input/output devices, and the input/output module.
inductive A load that uses a magnetic field generated by electricity. Inductive devices are likely to cause excess electrical noise.
input module The portion of a PLC that receives high-voltage electrical signals from input devices. The input module contains electrical conversion circuitry to reduce the voltage so it can safely send a signal to the CPU.
input/output devices I/O devices. A component that connects to an input/output module on a PLC and performs an action. Each input/output device, or field device, has its own electrical requirement.
input/output module I/O module. The portion of a PLC that contains connection points for input/output devices and circuitry to convert electrical current. An input/output module may be input only, output only, or a combination.
input/output points I/O points. A connection point on an input/output module for a field device wire. Input/output points allow an electrical connection between field devices and the input/output module.
isolated An electrical circuit connection that protects components from electrical damage. Electrical isolation blocks DC signals between circuits but allows AC signals to pass.
limit switches A type of contact that detects the presence or absence of an object. Limit switches may be used as discrete input devices.
line filters An electronic filter placed between an electrical component and an external power line. Line filters can reduce electrical noise and interference in a power line.
micro PLC A type of PLC with 32 or fewer input/output points. Micro PLCs are small, but they are still able to offer process control capabilities.
milliamperes mA. A unit of current equal to one thousandth of an ampere. Milliamperes may be used to measure current of analog input/output devices.
milliseconds ms. One thousandth of a second. Milliseconds can be used to measure the response times of programmable logic controllers.
modular PLC A type of PLC that has removable and interchangeable units, including the central processing unit, power supply, and input/output modules. A modular PLC can process a larger variety of input/output signal levels to meet the needs of a more complex application.
motor starter A mechanical device that turns on or off to start or stop a motor. Motor starters may be used as discrete output devices.
negative terminal A connection point on a battery or other voltage source that has more electrons than the positive connection point. Electrons flow from the negative terminal through the external circuit to the positive terminal.
output module The portion of a PLC that receives low voltage output signals from the CPU. The output module contains electrical conversion circuitry to increase the voltage it sends to output devices.
overcurrent Excess current flowing through a circuit. Overcurrent in an output module can be prevented by using a fuse or circuit breaker.
override Suspends the normal functionality of a device. An override can be activated using a force command.
PLCs Programmable logic controllers. A processor-driven device that uses logic-based software to provide electrical control to a machine or process. A PLC can replace many physical relays and hard-wired connections in a process.
positive terminal A connection point on a battery or other power source toward which electrons flow through the external circuit. The positive terminal's position on a PLC power source determines the direction of current flow.
power supply The component that provides power to the central processing unit and input/output modules on a PLC. The power supply sends low voltage, direct current to the CPU and input/output modules.
pressure transducers A device that converts variations in pressure into an analog electrical signal. Pressure transducers are a type of analog input device.
process control An approach to manufacturing that attempts to closely monitor and control all aspects of a process. Process control is provided by PLCs.
program The instructions entered into a CPU that direct the input/output devices to perform actions. A program contains lines of instruction.
programmable logic controller PLC. A processor-driven device that uses logic-based software to provide electrical control to a machine or process. A programmable logic controller can replace many physical relays and hard-wired connections in a process.
programming device A peripheral device connected to a PLC that someone uses to enter a program into a CPU's memory. Programming devices include personal computers and handheld programmers.
proportional control valves A control valve used in fluid systems that can change its output in response to an input value. A proportional control valve is a type of analog output device.
proximity switches A type of sensor that can detect the presence or absence of nearby objects without any physical contact. Proximity switches may be used as discrete input devices.
push buttons A type of contact that turns on or off to control some aspect of a machine or process. Push buttons are common discrete input devices.
relays A device that controls an electrical circuit by opening and closing contacts in another circuit. Relays are common discrete output devices.
resistors A device that controls current by opposing the flow of current. Resistors also help to reduce electrical noise in a power line.
run mode A CPU operating mode that is active when the CPU is executing the steps of a program. Run mode is required when using force commands.
sensors A device that detects a change in a physical condition and turns it into an electrical signal. Sensors are common analog input devices that respond to variable conditions, such as pressure, temperature, or flow.
shielded cable Cable that has an outer layer of insulating material covering an inner layer of conducting material. Shielded cable is used to reduce electrical noise in power lines.
sink/source A wiring configuration that allows current to flow in both directions. In a sink/source circuit, all shared common terminals are wired as sinking only or sourcing only.
sinking A wiring arrangement on a DC circuit in which the I/O device provides current to the I/O module. In a sinking configuration, the I/O device is the source of current.
snubber circuit An electrical circuit that absorbs energy and suppresses voltage spikes. A snubber circuit is used with inductive devices.
solenoids A device that converts electrical energy into linear mechanical force. Solenoids are common discrete output devices.
sourcing A wiring arrangement on a DC circuit in which the I/O module provides current to the I/O device. In a sourcing configuration, the I/O module is the source of current.
status LEDs An indicator light made of a semiconductive material that indicates a device's status or mode. Status LEDs on the input/output module show the condition of all connected input/output devices.
terminal block An electrical connector in which the device wires are clamped down to connection points. Terminal blocks allow many input/output devices to connect to an input/output module.
transducer A device that converts variable physical quantities into analog electrical signals. Transducers are used with analog input devices.
two-state Existing in one of two states at any given time. Two-state devices for a PLC send discrete signals that the CPU processes using binary digits.
variable frequency drive VFD. A device that converts incoming AC power into other desired frequencies to allow for AC motor speed control. A variable frequency drive is a common analog output device.
volts V. The unit of measure for electromagnetic force or pressure. Volts are an indication of the amount of current flowing through a circuit.