SDN Normal Operation
2-10
SDN Normal Operation
This section contains the SDN data timing specifications and a description
of the instrument status message.
SDN Data Timing Specifications
During normal operation SDN digital data travels on the SDN at 3.6 Mbit;
the bit cell width is 1/3,600,000 seconds, or 277.78 ns.
Therefore, the 12-bit word is 3.33 µs long. One poll cycle lasts 32 ms, but
not all of this time is available to transmit data. Considering all of the
overhead time (see breakdown below), approximately 7700 words of data
can be transmitted in every poll cycle. Overhead time consists of the
following:
a. Sync tap (10
µ
s)
b. Dead time (4 ms)
c. Time to listen for activity on each branch before sending a talk tap
(3.33 µs per branch)
d. Talk taps (10
µ
s per branch)
e. Time to listen for silence before going on to the next branch (12.22 µs
per branch)
f. Headers and delimiters in messages
Instrument Status Message
Once every system cycle (approximately once a second) each instrument
connected to the SDN broadcasts a message called the Instrument Status
Message. The content of this message allows all other instruments to
identify its operational status. Contained in every Instrument Status
Message are the following:
10101
277.78ns
Bit Cell Width
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