M87X Irig-B Time Qualifier; Determining The Correct Year - Novatech Bitronics M87X Series Manual

year. The control bits and straight binary seconds portion of the IRIG pulse stream are
ignored. The M87x transducer compares its present time to the IRIG time and stores the
delta time error. These errors are calculated every IRIG frame (every second) and are
accumulated into a sample buffer until the sample buffer is full. Once the buffer is full, the
buffer is passed to the IRIG Time Qualifier.
5.7.3c M87x IRIG-B Time Qualifier
The M87x IRIG-B Time Qualifier processes the sample buffer of time errors from the IRIG-
B Decoder. If the IRIG-B Time Qualifier detects several sequential time errors greater than
3 seconds, the IRIG-B Time Qualifier forces the M87x to immediately "jam" its clock to the
present IRIG-B time.
If the time errors are less than 3 seconds, the IRIG-B Time Qualifier examines all the
errors in the sample buffer. The error data is subjected to various proprietary criteria to
determine an accurate time offset. If the sample buffer does not meet the qualifying
criteria the sample buffer is discarded and no clock correction is performed. The IRIG-B
Time Qualifier continues to examine and discard sample buffers from the IRIG-B Decoder
until it finds one that meets the accuracy qualifications.
Once a sample buffer is qualified, the IRIG-B Time Qualifier calculates a clock correction
value and slews the M87x's clock to match the IRIG-B time. The slew time depends on
the magnitude of the clock correction. The time required to slew the M87x's clock to match
the IRIG time is approximately 30 times the clock correction value.
Slewing the clock ensures that time always moves forward. The clock may speed up or
slow down to attain proper synchronization, but it never moves backward. This ensures
that the ordering of events is always preserved while changing the clock. Ordering of
events cannot be guaranteed when the clock is jammed.
The IRIG-B Decoder does not sample the IRIG bit stream and build a sample buffer while
the M87x clock is slewing. All IRIG frames received during the M87x's clock slew are
ignored until the slew has completed.

5.7.4 Determining the Correct Year

The IRIG-B standard provides days of year, minutes of day, and seconds of minute
information. The IRIG standard does not provide any year information. IEEE-1344
specifies a bit pattern that is encoded into the IRIG control bit steam that specifies year
information. The M87x IRIG driver is capable of decoding the IEEE-1344 year information
from the control bits when connected to an IEEE-1344 compatible IRIG master. If the IRIG
master that is connected to the M87x is not IEEE-1344 compatible, the IEEE-1344
compatibility configuration switch in the M87x COM port configuration should be turned off.
This will prevent the M87x from incorrectly interpreting the control bits as year information.
If the IRIG master is not IEEE-1344 compatible, the M87x assumes that the year stored in
its non-volatile capacitor backed-up clock is correct. If the M87x back-up clock fails or the
M87x's year is incorrectly set, the IRIG-B Driver will assume that the year is the year
reported by the M87x's clock.
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ML0021 August 15, 2018 Copyright 2018 Bitronics, LLC