Abstract: Data to be communicated over a plurality of channels is divided into blocks (A1,B1,C1,D1,A2,B2, . . . ), and each successive block is transmitted along a different channel (A,B,C,D) on a cyclic basis. A 5-to-6 bit coding scheme is used with some data values having one possible encoding, with three binary ones, and the remainder having two possible encodings, one with two binary ones and the other with four binary ones. To maintain d.c. balance on each channel, two-ones and four-ones encodings are used alternately for successive occurrences on a channel of any data value having dual encodings. To assist detection of single-bit errors affecting the encoded data, a delimiter having one of two possible values is sent after the final block on each channel; this delimiter value is chosen in accordance with which encoding would have been selected if another dual-encoding data value had occurred on that channel.