1. A signalling system for communicating messages via a communications medium between a primary station and a plurality of remote stations in a data communications system, said signalling system including a variable length message which is transmitted between an originating station and received by at least one destination station, which includes a bit synchronization field, a message synchronization field, and a plurality of channel data blocks being comprised of: the first and second channel data blocks including an information field including control data and a check word, a parity field coded from the information field according to a predetermined code, and the bits of the fields being interleaved such that consecutive bits of a field are separated by a predetermined number of bits; and the third and following channel data blocks including an information field including data, a parity field coded from the information field, and the bits of the fields being interleaved such that consecutive bits of a field are separated by a predetermined number of bits, and the information field of the last channel data block further including a check word coded from the data in the information field of the third and following channel data blocks.