Importance

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Abstract

An asynchronous system for transmitting digital data from a data origination station to a data utilization station over a communications path such as an ordinary long-distance telephone network is disclosed. The system includes circuitry for synchronizing and maintaining synchronization between the receiving circuits and the transmitting circuits and for correcting errors introduced by the communications path. Each data word transmitted has associated therewith a synchronization code comprising a stop bit and a start bit. During normal operation, the system is synchronized by detecting the start bit which precedes the information portion of each data word. Synchronization is maintained for several data words even though the start bit is not detected due to interference, for example, by an auto-synchronization signal which includes a pulse positioned within the time period during which the start bit would normally be detected. Additionally, should a predetermined number of successive data words arrive at the receiver with errors, the auto-synchronization signal is disabled based on the assumption that the errors are due to a loss of synchronization. The transmitter circuits then transmits two synchronizing words, each containing the synchronization code with all other bits having a logic 'zero' level. The synchronization code of the two synchronizing words resynchronizes the system thereby restoring normal operation. That is, the system maintains normal synchronization even though interference destroys the start and/or stop bits of a number of successive words. Should this interference continue such that the automatic synchronization circuits can no longer maintain synchronization, the auto synchronization circuit is disabled and normal synchronization is restored by transmitting two special synchronization words, all without any loss of data.

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First Claim