IEEE 1344

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IEEE 1344 defined how to encode synchrophasor data for power systems using IRIG-B time codes. It added year, time quality, daylight saving time, local time offset, and leap second information to the IRIG-B code. IEEE 1344 was superseded by IEEE C37.118 in 2005, and the time extensions were adopted into the IRIG timing standard in 2004.

IRIG-B timecode consists of 100 bits that repeat each second, with every tenth bit serving as a position identifier. Bits 60–68 and 70–78 are reserved for other uses; IEEE 1344 used them for such a purpose.

The standard defines several fields:
- DST and leap warning: set no more than 59 seconds before the change and indicate the change at the end of the minute.
- Leap seconds: the warning bit is set, the seconds field shows 60, and the Straight Binary Seconds field equals 60 + 60×minutes + 3600×hours. The next second clears the warning and the SBS repeats. The LS bit is always 0 (negative leap seconds have not occurred and are considered unlikely).
- Clock quality: a binary value; 0 means UTC-traceable but no specific accuracy; 15 means the clock has failed. Values 1–11 indicate very high accuracy, down to UTC±1 ns (x=1) up to UTC±10 s (x=11).
- Time zone offset: the difference between UTC and the timecode. UTC = timecode + offset, and this offset changes with DST. The SBS field also jumps by 3600 seconds when DST is active.
- Parity: an even parity bit for data bits 1–74.
- Marker bits: ignored.

The standard also added two digits for the year in bits 50–58, an addition adopted by IRIG in 2004.

IEEE C37.118 was designed to replace IEEE 1344. It uses a section-based format with ordinary binary integers instead of IRIG and BCD, and includes separate parts for synchrophasor performance and frequency of measurement. The 2005 version, C37.118-2005, uses the opposite sign convention for the time zone offset: offset = timecode − UTC, so UTC = timecode − offset.