The digital I/O board is an optional card which can be added to 6800, 5900, 5500A, and 5500 Series frames. Digital communication, unlike analog, does not sense continuous signals but inputs or outputs of one of two states: high or low, open or closed. The card used in conjunction with the frame and Bluehill^® software allows the operator to trigger external or internal events. It allows for 4 input and output channels to be monitored and/or triggered simultaneously.