Allowing quick and reliable analysis of USB data transfers, the USB Agent is hooked into the data bus at any point and operates as a protocol analyzer while logging the complete data flow into a buffer as large as 8 MB. A host PC connects the USB Agent via an LPT interface (SPP or EPP) and controls the recording and analysis of the data flow. The HiTOP/win user interface runs on any 32-bit Windows operating system. A real-time capable trigger system controls the recording, where pre- and post-triggers can be established. Trigger conditions include: data patterns; events such as user-definable PIDs; and PID combinations with address and end-point or error conditions. Unit can be used to search for these events within the recorded data flow or to filter the display.