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.

Switches are pretty straightforward to wire. For security reason switch contacts are always drawn in the OFF position when in systems initialize. In the case of rotary switches the wiper arm will always be connected to the first contact, which will be the off position.