A vital communication system connects an input location to an output location using a non-vital communication link. In addition to transmitting, from the input location to the output location, a word with a bit dedicated to each different input, a system check word is transmitted. Each different input is assigned a unique multi-bit value, and the system check word comprises a linear combination of the unique multi-bit values for each of the inputs in a given binary state. Thus, there is a relationship between the transmitted words with bits dedicated to each input line and the system check word. At the output location, this correspondence is checked for. In addition, a further check is employed which first produces an output check word made up of a linear combination of the unique names for each output which are not in the given binary state. This output check word is then linearly combined with the received system check word. In the absence of a failure, the result of this linear combination of the output check word and the system check word should be constant regardless of the particular condition of different inputs and outputs, so long as the output and inputs actually correspond. Apparatus is provided to check for this constant result.