PURPOSE: To prevent the malfunction of a personal computer by controlling the output state of input data from an input device by a control signal from a liquid crystal control circuit and transmitting data to an arithmetic circuit only when a liquid crystal display device or a CRT display device is in a display possible state.

CONSTITUTION: When count terminates in the arithmetic circuit 6, the arithmetic circuit 6 sets a timer state signal 3 in a count termination state and transmits it to the liquid crystal control circuit 7. When the timer state signal 3 in the count termination state is inputted, the liquid crystal control circuit 7 sets a back write on/off signal 4 as an off state and transmits it to the liquid crystal display device and an input signal control circuit 5. When the back write on/off signal 4 in the off state is inputted, the liquid crystal display device puts outs a back light. The input signal control circuit 5 inputting the back write on/off signal 4 in the off state stops the conversion/transmission of input data and outputs only a counter reset signal.