Abstract: A new method for constructing keyboard circuit, namely, completely combinatorial keyboard circuit, was proposed. The new key board is composed ofnbidirectional interfaces based on the principle of permutation and combination. It allows the maximum number of 2n-1+n (2n-1- 1) keys to be implemented on thenbidirectional interfaces, much larger than the conventional matrix key board. Its components can also be utilized to construct key board circuits according to practical requirement.