Microprocessor data input switching circuit - uses interrogated switch groups connectable to multiple channel data bus

A circuit delivering input data to a microprocessor circuit has switches, esp. keyboard switches, which place switching signals onto a data bus connected to the microprocessor and having several data channels. The switches are connected to a common reference potential. It requires only a few channels connecting the switches to the circuit and includes a facility whereby a user connection signal is returned. The switches are arranged in groups of a number equal to the number of data channels being used. Interrogation pulses activate the switch groups.