PLC I/O Devices

PLC Input and Output Devices

The term I/O refers to Input/Output. I/O is information representing the data that is received from sensing devices and the commands that are sent to actuating and indicating devices. The I/O System is the collection of physical elements of the control system that either provide or use I/O data. There are two major types of I/O:

  • Digital Device - binary devices which must be in one of only two states: on or off.
  • Analog Device - continuous devices – sense and respond to a range of values.


Digital IO

Digital input devices are binary – they may be either on or off, open/closed. Common digital field input devices include pushbuttons, limit switches, and photoeyes. Common digital output devices include relays, motor starters, and solenoid valves.

Analog IO

Analog input devices sense continuous parameters. The information that they provide is given as a continuous range of values, not just an on or off indicator. Common analog inputs are pressure, temperature, speed, etc. Analog output devices respond to a range of output values from the controller. Common analog output signals include motor speed, valve position, air pressure, etc.

I/O modules connect “real world” field devices to the controller. They convert the electrical signals used in the field devices into electronic signals that can be used by the control system, and translate real world values to IO table values.

I/O modules communicate with PLC CPU in one of three ways:

  1. Backplane (or Rack) – The I/O modules can be located in the same rack or station. Communications then takes place within the rack or across the backplane.
  2. Backplane extension – backplane extension modules allow I/O modules to be located in racks or stations which are separated from the controller.
  3. Device network – modules can communicate with a controller over a network. Industrial networks are used to interconnect field level devices with controllers. Common IO networks include Profibus, ProfiNet, DeviceNet, Fieldbus, and others.


“Point Count” and the Size of the PLC System

The number of I/O devices used within a control system is called its “point count”. Analog device data requires significantly more manipulation and processing than digital device data. Digital and analog point counts are typically considered separately. The total number of digital and analog points is used to give an indication of the size of a control system.