PURPOSE: To enhance the reliability by doubly providing the controllers equipped with both the test input/output function and the peripheral interruption function, between CPU and the peripheral equipments.

CONSTITUTION: At the time of the process input/output by CPU, in case when abnormal has been detected, the test input/output are executed by use of the test input/output functions 7a, 7b of the controllers 4a, 4b from CPU5, and if the test input/output are normal, it is decided that the controllers 4a, 4b are normal and the process input/output units 1W3 are abnormal. On the other hand, if the test input/output are abnormal, it is decided that the process input/output units 1W3 are normal, and the controller 4a or 4b is abnormal, and thereafter, CPU executes the processing by the other controller without using the abnormal controller.