A data bus system suitable for use on board an aircraft for control of power switches, for example. Data is transmitted asynchronously between a bus controller (1) and a plurality of remote terminals (2) which are linked to the data bus (3) by transformer couplers (4). Data is modulated onto a carrier for efficient transmission via the couplers (4). Asynchronous operation ensures use of a simple protocol and inexpensive interface hardware. <IMAGE>
