Decoder for self-clocking serial data communications

DETAILED DESCRIPTION To attain these and other objectives, at the receiving end of at least one serial data channel to which a receiving device connects, this invention provides simple but highly efficient and effective receiving interface circuitry for such a link.
The interface circuitry comprises and centers around a decoder for connecting to the channel and separating from the composite signal on the channel the component data bits and clocking signals; a carrier detector, also adapted for connecting to the channel and for detecting the presence of information on the channel, and in response thereto, for enabling a serial shift register to receive serially the transmitted data bits in response to the clocking signals; and an internal clock synchronizer for synchronizing the occurrences of parallel transfers of data bits to the receiving device with the occurrence of a predetermined number of serial data bits at the beginning of the transfer of serial data from the channel.
The decoder is elegant in its simplicity.
It employs a flip-flop, an exclusive-OR gate, and a minimum of two delay elements.
the delays cause the flip flop to sample the composite signal about one-quarter way through each bit cell.
The carrier detector is also quite simple.
The bus signal is compared against a preset threshold.
If the bus signal exceeds the threshold, a pulse train is generated.
The pulses set a latch.
The trailing edge of a lower frequency internal clock causes the latch to clear if the bus signal vanishes.
An internal clock is provided, independently of the clock derived from received data.
One time at the beginning of each message, the synchronization character is detected in the incoming message, and the phase of the internal clock circuit is corrected.
It is this clock which controls the transfers from the interface to a host device.
Other objectives, advantages and features of the invention will become apparent upon review of the succeeding description of an illustrative embodiment taken in connection with the accompanying drawings