Fault detection hardware is employed at the I/O interface racks of a programmable controller to detect fault conditions which may occur there. An I/O fault line is connected in daisy chain fashion between the I/O interface racks and the controller processor, and when a fault is indicated at any of the I/O interface racks, it is communicated to the controller processor through this line. The indicated I/O fault freezes, or holds, a microprocessor in the controller processor, and if the I/O fault persists for a preselected time interval, all operating devices connected to the programmable controller interface racks are decontrolled. A watchdog timer is also provided in the controller processor for detecting malfunction conditions which may occur at the processor. When such a condition is detected, the operating devices connected to the programmable controller interface racks are decontrolled.