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 - binary devices which must be in one of only two states: on or off.
- Analog - continuos devices - sense and respond to a range of values.
Digital IO
Digital input devices may be either on or off; they may not hold
any other values. For example, digital position sensors do not
sense how close an object is, they only tell if the object is within a
range of positions. 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:
- Backplane - The I/O modules can be located in the same rack or station. Communications then takes place within the rack or across the backplane.
- Backplane extension - backplane extension modules allow I/O modules to be located in racks or stations which are separated from the controller.
- 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 are FieldBus, Profibus, and DeviceNet.
"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.