Roughly described, a method of controlling transportation of debug data on an integrated circuit chip. The chip has a shared hub and a number of peripheral circuits. Each peripheral circuit is connected to a respective debug unit, and between each debug unit and the shared hub there is an interface configured to transport data messages over each of a number of prioritized flows. In the method, still roughly described, control data for controlling the state of a debug unit is transported on a priority flow having a first priority, and debug data output by a debug unit as a result of debugging the peripheral circuit connected to that debug unit is transported on a flow having a second priority, the first priority being higher than the second priority.