Abstract: In a data communication system an interface unit connects a device to a common communications channel shared with other devices using e.g. a carrier-sense multiple-access protocol. Individual bytes for a predetermined destination device are supplied by the device to communications controller 6 without any grouping into packets. They are then stored in a RAM 8 under control of a microcomputer 5. As soon as a byte is received, the interface unit attempts to transmit a packet; when the right to transmit is won the interface unit transmits a packet containing that and any subsequent bytes. The length of the packet therefore depends on the delay before a packet can be transmitted, and hence on the loading on the channel.