Appendix B
IRIG Protocol Overview
©
National Instruments Corporation
B-5
NI PXI-6682 User Manual
The NI PXI-6682 uses the time of day information transmitted as BCD to
synchronize its internal timebase. If the IRIG-B signal includes the year,
then it also uses that information to synchronize its clock. Otherwise, it gets
the year from the host computer. The NI PXI-6682 disregards the rest of the
information contained in the IRIG-B signal. Therefore, when configured to
synchronize to IRIG-B AM, the NI PXI-6682 supports IRIG-B 12X, and
when configured to synchronize to IRIG-B DC, it supports IRIG-B 00X.
The following assumptions are made regarding the received IRIG-B signal.
All conditions must be met for the NI PXI-6682 to be able to synchronize
accurately:
•
Seconds begin every minute at 0, increment to 59, and then roll-over
to 0.
•
Minutes begin every hour at 0, increment to 59, and then roll-over to 0.
•
Hours begin every day at 0, increment to 23, and then roll-over to 0.
•
Days begin every year at 1. Days increment to 365 in non-leap years,
or to 366 in leap years, and then roll-over to 1. Leap years must be
supported. Valid values for year are 01–99, inclusive. Years are
assumed to be in the XXI Century. For instance, year
07
represents
2007. If the year is not supplied (sent as
00
), the OS system time is
read and the year is derived from it.
To achieve proper synchronization of the NI PXI-6682 ensure that the
IRIG-B source used conforms to the requirements listed above. Note that
most IRIG-B sources conform to these requirements.
90–97
Eight most significant bits of time of day in straight
binary seconds (bit 90 -> 2
9
… bit 97 -> 2
16
)
99
Position identifier P
0
Bits not listed are index markers, and are sent as binary zeroes.
Table B-3.
IRIG-B Bit Assignments (Continued)
Bit
position
Information transmitted
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