In accordance with the present invention a system for detecting transaction errors in a system comprising a plurality of data processing devices using a common system interconnect bus, comprises a node controller operably connected to said system interconnect bus and a plurality of interface agents communicatively coupled to said node controller. Error corresponding to transactions between said interface agents and other processing modules in said system are directed to said node controller; and wherein transaction errors that would not normally be communicated to said system interconnect bus are communicated by said node controller to said system interconnect bus to be available for detection. In an embodiment of the present invention, the interface agents operate in accordance with the hypertransport protocol. A system control and debug unit and a trace cache operably connected to the system bus can be used to diagnose and store errors conditions.