They are the numbers 0 and 1; each comes from binary notation, where 0 is off and 1 is on.

I/O can also stand for Input / Output. In todays world it would be more likely to be understood by using the ( circle with an electrical symbol inside for ON ) and the( circle with the slash through the electrical symbol for OFF ).

Respectfully Yours,

Richard W Clothier

PO Box 25,Legal,

Alberta, T0G1L0.