A multimedia protocol interface for 64K Kbit/s data flow, organized accorg to the frame structure specified in CCITT Recommendation H221 and transmitted and received according to the standard 64 kbit/s protocol or 256 kbit/s 10 M protocol, adapted to operate autonomously or aided by an external microprocessor and has a receiver, a transmitter, a control block and circuitry for the local test of the operation of the interface itself. The external microprocessor can read both the received data and the information relevant to the receiver conditions, and can also send data to be transmitted or can read information relevant to the transmitter conditions. The receiver receives the data and clock and synchronism signals and emits at the output the received data, associated with timing signals useful for the external devices connected to the interface for data recovery. Analogously, the transmitter emits the data, receives clock an synchronism signals and supplies data and timings to the external devices. The local test is carried out by verifying whether the receiver succeeds in recovering the synchronism from the signal emitted by the transmitter if the same interface, under microprocessor supervision, and if the data are correctly transmitted and received.