A data processing apparatus having processing circuitry and debug circuitry is debugged by operating the processing circuitry to generate data. The debug circuitry is employed to generate trace elements indicative of the operation of the processing circuitry. Trace elements are caused to be output from the data processing apparatus over a communication bus capable of connecting a plurality of devices. The communication bus is controlled by a protocol for data interchange requiring data interchange from any device on the communication bus to be controlled by a single processing system. The passing of the trace elements onto the communication bus is controlled using an interface unit of the debug circuitry. The interface unit comprises a controller arranged to allow each of the interface unit and processing circuitry to be separate processing systems which can each independently control data interchange from the data processing apparatus.