| 摘要 |
PCT No. PCT/US81/00182 Sec. 371 Date Feb. 11, 1981 Sec. 102(e) Date Feb. 11, 1981 PCT Filed Feb. 11, 1981 PCT Pub. No. WO82/02809 PCT Pub. Date Aug. 19, 1982.A data communications system which includes a plurality of terminals TL1-TL5) and a data bus (DB), wherein each of the terminals has autonomous access to the data bus. Each terminal includes a receiver (114) which is normally enabled to receive messages on the data bus and which provides a signal (ADA) signifying the absence of messages. A transmitter (120) within each terminal includes a protocol control unit (140) which enables the transmitter to initiate message transmission only upon (i) the detection of an absence of messages on the data bus whose duration substantially equals that of a sync gap that is common to all of the terminals, and (ii) the subsequent detection of an absence of messages on the data bus whose duration substantially equals that of a terminal gap that is unique to the terminal, with the duration of the sync gap being greater than the duration of any terminal gap. Within each protocol control unit, the sync gap is detected by a sync gap timer (160, 162, 164, 166, and 168) and the terminal gap is detected by a termial gap timer (190, 192, 194, 196, and 198) which is enabled (169) by the sync gap timer, with both timers being responsive to signal ADA. In order to limit the rate of message transmission by any terminal, each protocol control unit may also include a transmission interval timer (170, 172, 174, 176, and 178) which enables message transmission only upon the further condition that (iii) a transmission interval has elapsed from a previous message transmission by the terminal, with the duration of the transmission interval being substantially the same for each terminal.
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