Abstract: A JTAG testing system comprising a JTAG test equipment and a device under test compatible with the JTAG configured to form a synchronous data transmission connection with one another for transferring test data through a transmission path placed between predetermined interfaces. An asynchronous connection can be used as the transmission path between the interfaces, whereby the test equipment and said device under test are provided with a transceiver on the transmission path side configured to arrange the test data to be sent from said interface into a mode appropriate for an asynchronous transmission path and correspondingly to arrange the test data to be received from the asynchronous transmission path into a synchronous mode required by said interface.