Abstract: An interface circuit (10) for coupling a parallel data device (12) to a serial data channel (14, 16) over which Manchester-type codes are transmitted. In the interface circuit, an efficient and reliable Manchester decoder (22), comprising a flip-flop (50), an exclusive-or gate (52), and at least one delay line (58A or 58B) separates the data and clocking signals. The serial data signals are clocked into a serial register (30) under control of the external clocking signals from the channel. A carrier detector (24) enables the serial register only when valid information signals are present. A parallel data register (40) receives in parallel the data from the serial data register. To get in phase the external clocking signals with the internal clock source, an internal clock synchronizing circuit (34, 42) recycles the internal clock source upon the occurrence of a synchronizing character that is transmitted over the serial data channel.