We describe techniques suitable for communicating switching data in a control system controlling, for example, kilovolts at hundreds of amps. The system comprises a central controller (110) coupled to sub-controllers (120) and thence to a plurality of switching device controllers (SDs,130), preferably in a tree-structure, each SD controlling a power semiconductor switching device such as an IGBT. The method includes formatting switching control data as one or more switching control data packets comprising data for controlling switching of a combination of the devices, sending these from the central controller to the switching device controllers, at the SDs formatting state data representing states of the switching devices controlled in combination into a plurality of acknowledgement data packets, and sending these back to the central controller. The techniques may be employed for controlling the synchronised switching of potentially, tens, hundreds or thousands of power semiconductor switching devices.