SEQUENTIAL CONTROLLER

Disclosed is a sequential controller in which the step of control sequence for controlling various process control devices is advanced by a step advance unit basing on the time count by an answer-back timer. The controller includes an input check unit in which input collating check units provided for individual inputs to the control devices implement individual checking at each control step so as to detect abnormality of the control devices or abnormality of input to the related internal sections. Each input collating check unit compares the alloted input signal with the input expected value at the current control step retrieved from an input table in accordance with the input signal, and, in case both values are not consistent over the entire range at that step, it issues an alarm output if the input expected value at the current control step is consistent with the input expected value at the preceding control step, or it issues an alarm signal upon expiration of the answer-back timer if both input expected values are not consistent. The controller skips the answer-back checking when input expected values of each input at two consecutive control steps are equal, thereby preventing the malfunctioning in the input checking operation.