A communication channel for conveying quasi-static signals representing several alternative states one of which is potentially unsafe, e.g. the control signal for a signal lamp in a railway control system. Each channel has a signature code (Figs. 1 and 2) which is continuously repeated in different forms, e.g. non-inverted (Fig. 2(a)), inverted, or alternately inverted and non-inverted (Fig. 2(b)), to represent different states. When a potentially unsafe state is indicated, the repeated signature is periodically corrupted according to a predetermined error pattern. The channel receiver (Fig. 4) is arranged such that its output (32,34) can only be held in the unsafe state providing both the signature and the error pattern in the received signal are correct.