• 15 parts
  • Checked September 2026

Parts of a PLC

A modular programmable logic controller from the front, then wired to a machine.

Parts of a PLC

Name Date

Tap any part to read, rename or leave it out

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
More options

Write the name of each numbered part.Answer key

  1. Rack (chassis)
  2. Power supply
  3. CPU (processor)
  4. Status LEDs
  5. Mode selector (keyswitch)
  6. Battery
  7. Input module
  8. Output module
  9. Analog module
  10. Terminals
  11. Communication module
  12. Backplane
  13. Programming device
  14. Sensor
  15. Actuator

Drawn for this site on 2026-09-30 from plain geometry. Licence: own work.

Each numbered ring is a button. Use the arrow keys to move between them and Enter to choose. Names are read out in Study, and once a part is answered.

Next step Next: the parts of a circuit

All 15 parts on the diagram

Show the list

The controller 12

  1. Rack (chassis) It has room for a modular PLC's power supply, processor and modules. Those are the input, output and communication modules the job needs.
  2. Power supply It turns alternating current from the mains into low direct-current voltage. The modules run on it.
  3. CPU (processor) Reads the inputs, works through the control program and sets the outputs. The program is held in its memory.
  4. Status LEDs One lights when the program is running. Another lights when the CPU has stopped, another when there is a fault.
  5. Mode selector (keyswitch) One maker calls it the mode selector switch. On another it is turned with a key and called the keyswitch.
  6. Battery Without it, battery-backed memory would lose its program in a power cut.
  7. Input module Each of its points is wired to a switch or sensor. It tells the CPU whether that device is on or off.
  8. Output module Each point switches one device on the machine on or off.
  9. Analog module Temperature, pressure, flow and weight usually reach a PLC as analog signals.
  10. Terminals On many modules the whole strip of terminals unplugs. A module can then be changed without undoing the wires.
  11. Communication module Through it the PLC exchanges data with other controllers and computers.
  12. Backplane It joins the modules to each other and carries power to them.

The controller at work 3

  1. Programming device It is joined to the CPU by a cable or a network. The screen here shows a ladder program.
  2. Sensor Limit switches and photoelectric sensors are common ones. Each reports on or off.
  3. Actuator Here it is a small motor. Together, sensors and actuators are the machine's side of the PLC's inputs and outputs.

How this list was compiled: Every name checked on 2026-09-30, a name kept only where two of these three use it: (1) Wikipedia, Programmable logic controller, https://en.wikipedia.org/wiki/Programmable_logic_controller, text only; (2) one maker's manuals, full text at https://archive.org/details/manualzz-id-989334, https://archive.org/details/manualzilla-id-5813256 and https://archive.org/details/manualzilla-id-5844774; (3) a second maker's manuals, full text at https://archive.org/details/manualzilla-id-7353703 and https://archive.org/details/manualzilla-id-6980774. The two makers name the mode switch differently, so the label gives both names. The detail lines are in our own words. Licence: own work

The drawing: Drawn for this site on 2026-09-30 from plain geometry; no maker's controller, nothing traced. Licence: own work

How it works

A modular PLC in two drawings. The first is the controller from the front, module by module. The second shows it at work: a programming device, a sensor wired in and an actuator wired out.

Read the full guide

Read the rack from left to right

The power supply comes first, then the CPU with its status LEDs, mode selector and battery. Next are the input module, with wires coming in, and the output module, with wires going out, then the analog module and the communication module. Wires from the machine end at the terminals. The empty slot shows the backplane that every module plugs into.

  • Inputs and outputs are told apart by direction: a sensor sends a signal in, an actuator is driven by a signal out.
  • The scan never stops: read the inputs, run the program, write the outputs, then start again.

Questions and answers

What are the main parts of a PLC?

A processor (CPU) that runs the control program held in its memory, a power supply, input and output modules that connect it to the machine, a communication interface, and a programming device for writing the program and loading it.

What is the difference between an input module and an output module?

An input module brings signals in from the machine's switches and sensors. An output module sends signals out to the devices that do the work. The CPU reads the first and sets the second.

What is a PLC scan cycle?

The loop a PLC repeats for as long as it runs. In its simplest form it has three steps: read the inputs, execute the program, write the outputs. One pass usually takes some tens of milliseconds.

What is a PLC rack?

The frame, also called a chassis, that holds the modules of a modular PLC. The modules plug into a backplane at the back of it. A large system can have several racks run by one processor.

More quizzes

Show 7 more

Browse Science Quizzes