The test apparatus (PC,KB,DIS,PR,IF) communicates via a standard serial bus (12) with a device under test (DUT) to verify conformity with a standard application protocol defining the format, meaning and applicability of messages passed via the bus (12). The apparatus generates a test sequence of commands and requests and analyses the replies of the DUT for conformity. The test sequence and the analysis are adapted automatically in response to previous replies of the device, to provide a thorough but efficient test of the device and any subdevices identified within it. Where the standard application protocol defines extended autonomous cooperation between devices, the test apparatus emulates one or more further devices to be controlled and interrogated by the device under test.