A communications device, particularly but not exclusively for use with a routing circuit is arranged to transmit and receive message packets. It comprises an output buffer (42) which operates to convert data and flow control information into a plurality of bit sequences and to transmit them at a predetermined frequency. An input buffer is operable to decode incoming sequences into data items and flow control information. The input buffer (44) can store the data items and count them and transmit flow control information to the output buffer. A method of encoding the bit sequences as balanced six-bit codes is also described.