Input-output interface circuit with multiplexers selecting an external signal or an internal output signal as the input signal from an I/O terminal

An input-output circuit structure interposed between input/output signal processing means and external input-output terminal means for transmitting an external signal from the external input-output terminal means to the input/output signal processing means as an input signal and transmitting an output signal from the input/output signal processing means to the external input-output terminal means as an external output signal, said input-output circuit structure comprising:

• first signal selection means having one input terminal and first and second output terminal for transmitting an output signal supplied from said input/output signal processing means to said output terminal of said first signal selection means;
• second signal selection means having one input terminal and first and second output terminal for transmitting an input signal to signal processing means through said output terminal of said second signal selection means;
• external output signal transmission channel means for connecting said first output terminal of said first signal selection means and said external input-output terminal means;
• internal signal transmission channel means for connecting said second output terminal of said first signal selection means and said first input terminal of said second signal selection means;
• and
• external input signal transmission channel means for connecting said second input terminal of said second signal selection means and said external input-output terminal means;
• said first signal selection means operating selectively in an external signal output mode in which said output signal is transmitted as an external output signal to said external input-output terminal means through said external output signal transmission channel means and an internal signal transmission mode in which said output signal is transmitted as an internal signal to said internal signal transmission channel means, and said second signal selection means operating selectively in an external signal input mode in which an external input signal from said external input signal transmission channel means is transmitted as said input signal and an internal signal output mode in which said internal signal from said internal signal transmission channel means is transmitted as said input signal.