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Synchronous Transmitter Receiver Clocking Method Disclosure Number: IPCOM000094738D
Original Publication Date: 1965-May-01
Included in the Prior Art Database: 2005-Mar-06
Document File: 3 page(s) / 40K

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Jordan, FE: AUTHOR [+1]


This system establishes and maintains synchronism between duplex telegraph terminals A and B operating in the synchronous mode.

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Synchronous Transmitter Receiver Clocking Method

This system establishes and maintains synchronism between duplex telegraph terminals A and B operating in the synchronous mode.

In synchronous mode transmission, a receiving terminal is synchronized with a transmitting terminal without the use of synchronizing elements, i.e., start and stop elements, to identify the beginning and end of a character. This is accomplished by providing character synchronization at the beginning of a message and by maintaining signal element synchronization throughout the message.

Assume that a character is comprised of eight signal elements and that each element may have one of two values, i.e., positive or negative DC level. A receive strobe for sampling the level of the received signal is generated by a frequency divider-oscillator combination 10. The frequency divider can be, for example, five triggers driven by an oscillator. Identical circuits are used in both terminals A and B so that reference to numerals shown in terminal A refer to similar circuitry in terminal B. The receive strobe is generated at the signal element rate and samples each element at the center of its time period.

Time comparator 24 determines whether the strobe pulse occurs off-center in the signal period. If the strobe is too early, an output from comparator 24 advances the frequency divider an appropriate amount to cause the strobe pulse in the next cycle to occur in the center of the signal element. If the strobe is late, the frequency divider is retarded accordingly. In this manner, the receive strobe is continuously adjusted to maintain its position in the center of the received signal element.

Characters eight elements long are transmitted end-to-end in a continuous stream. Appropriate non-data characters are provided to distinguish the beginning and end of a message and to establish character phase so that only eight-bit sequences comprising data characters are transferred to output device
22. Accordingly, each transmitted message is preceded by a start-of-record character (SOR) and is followed by an end-of-record character (EOR).

Assume that terminal A initially commences transmitting to terminal B, i.e., in half duplex. The receiver portion of terminal B must determine which of the signal elements received is the first element of a character, i.e., terminal B must establish character phase with terminal A. Initially, however, the strobe generated at terminal B may not be properly synchronized with the signal elements and some period of time is necessary for the frequency divider to be corrected to sample the signal elements at their midpoints.

To accomplish th...