Abstract: Vital transmission checking apparatus is disclosed for a communication channel which transmits quasi-static signals representing several alternative states of a function, e.g., the control function for a signal in a railway control system, one of which states is potentially unsafe. Each function channel transmits a digital signature code of predetermined length which is continuously repeated in a selected form, e.g., non-inverted, inverted, or alternately inverted and non-inverted, to represent the different states of the function. To provide a check of proper transmission, the repeated signature code is periodically corrupted in accordance with a predetermined error pattern which is repeated at a lower repition frequency than the code. The channel receiver is so designed that its output can only be held in the selected state when both the signature code and the error pattern in the received signal are correct. If a fault is detected, the receiver output is shifted to the most safe state of the function.