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.

Abstract: A programmable test point selector circuit for interfacing a test set with a system to be tested is disclosed. Typically, the system to be tested is provided with a multi-pin connector or receptacle to which the automatic test set is connected. The disclosed programmable test point selector circuit is interposed between the test set and this connector. Digital program words from the test set are used to designate pins of the multi-pin connector for use as either an input pin or an output pin. Additionally, the programmable test point selector permits at least one voltage level of the digital signal supplied to any pin designated as an input to be independently programmed over a wide range of values. An output sensing circuit is also provided which permits any individual pin or group of pins to be selected for monitoring to determine the voltage level of the signal present at the selected output pin relative to a threshold level.