PURPOSE: To obtain a data processor which is diagnosed in a short time, by connecting an input and output controller to respective input and output devices with a common selection signal line, and using each individual selection signal line as a diagnostic signal line for respective input and output devices.

CONSTITUTION: An input and output controller is connected to respective input and output devices through a common selection signal line, and each individual selection signal line is used as a diagnostic signal line for respective input and output devices. For example, the input and output controller 1 outputs a diagnostic signal 8, which is inputted to a driver 9 to generate a common selection signal. This common selective signal is inputted to stray preventing circuits 11-AW 11-N of the input devices 4-AW4-N through the common selection signal line 7 and when the input devices have no fault in the hardware, and NAND gate 16 sends out an output signal. Then, this output signal is inputted to the input and output controller 1 through individual selection signal lines 3-AW3-N and stored in a memory 10 when a diagnostic signal 8 inputted to the trigger terminal TRG of the memory 10 rises.