Abstract: A data processing system according to the invention comprising a group of at least a first and a second module, wherein each module has a data processing facility, a clock for timing data transmissions from the module to another module, a time-slot counter for counting a number of time slots which are available for transmission of data. The modules have a first operational state wherein the counted number of time slots is less than or equal to a predetermined number, in which operational state data transmission is enabled, and a second operational state wherein the number is in excess of the predetermined number, in which second operational state data transmission is disabled, Each module has a notifying facility for notifying when it is in the second operational state.