Many types of digital signals like switch closures, relay contacts, and TTL laver compatible can be read by digital I/O card directly. Some type of digital signals input may require signal conditioning like reduce higher-level voltage to TTL level. A variety of signal conditioning module is available to provide isolation and device circuitry protection functions.

Digital Output

Digital output provides functions for turning electric device ON or OFF. TheApplications are ranged from driving a relay to turning ON/OFF lamp solenoids, contactors, motors, or alarms etc. In application for latching outputs, a "1" typically causes the associated switch or relay to latch,while a "0" causes the switch to unlatch. Devices can be turned on or off,depending on whether the external device is need normally open (ON) or normally closed (OFF).

The TTL level signals can be used to drive 5-V relay coils; a protective diode is used to protect the digital output circuitry. Because data acquisition boards can typically supply only 24 mA of driving current, they are intended primarily to drive other logic circuits, not final control elements. Scaling may be needed so that logical voltage levels are sufficient to cause switching in larger relays. Outputs intended to drive larger solenoids, contactors, motors, or alarms also may require a booster.