| 摘要 |
A digital data communication system uses a common channel (22) to carry both normal messages and supervisory messages, the latter being inserted into the message stream during intervals between normal messages. Each transmitting location has a switch (16) with three settings. Setting 1 enables data to pass from the normal message generator directly to the channel. Setting 2 enables data to pass indirectly from the normal message generator through a delay line (X1-X2-X3) to the channel. Setting 3 enables data to pass from the supervisory message generator to the channel. A switch control (24) comprises message status detecting circuitry associated with the delay line. This switch control enables the switch to assume setting 3 when no normal message is being generated and a supervisory message is awaiting transmission. When the supervisory message ends, the switch is controlled to assume setting 1 if the delay line contains no significant normal message data nor any indication that an out-of-synchronism condition exists; otherwise the switch assumes setting 2 until the delay line is cleared of normal message data or reaches a synchronous state, as the case may be, whereupon it returns to setting 1. Each supervisory message has a distinctive format which enables it to be recognized at the receiving location and diverted from the message stream to a supervisory message receiver before it can enter the normal message receiver.
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