Distributed I/O

Input/Output (I/O) is central to every PLC. Input is the energy or information that enters a system and is usually related to sensors, switches, potentiometers, and so on. Output is the energy or information that leaves a system and is usually related to lighting devices, electric motors, alarms and so on. Both digital and analog I/O are fundamental in automation technology. Digital I/O is binary in nature and can either be ON or OFF. Digital input devices are push-buttons, light switches and proximity switches. Digital output devices are relays, motors starters and lights. Analog I/O is a time-varying signal and has a range of values. An analog input into the PLC can increase or decrease in very small increments and the PLC can produce an analog output that acts the same way. Analog input devices are pressure sensors and temperature sensors. Analog output can be used to control the power output on a generator or the position of a needle on an analog pressure meter. 

Distributed I/O systems include small field devices with a wide range of I/O options. This can include counter inputs, digital and analog channels and temperature measurements. These modular devices provide a flexibility lacking in traditional devices, like a PLC, that require a high density channel count and long cable runs. Distributed I/O systems reduce wiring and installation costs as well as allowing for quick machine setup and easy expansion. Distributed I/O can be employed in a physically local arrangement with control and I/O in a single enclosure. This setup can provide diagnostics, modularity and short-circuit protection. Other applications may use both rack-mounted and distributed I/O. Rack-mounted I/O supports many points requiring fast response and local devices where wiring is minimal, while distributed I/O connects some remote devices. Systems that extend over a large area may use rack I/O for local control panel indicators, push buttons and related devices while using a distributed system for the remote devices.