IRIG Timecode - Time Code Structure

Time Code Structure

IRIG time code is made up of repeating frames, each containing 60 or 100 bits. The bits are numbered from 0 through 59 or 99.

At the start of each bit time, the IRIG time code enables a signal (sends a carrier, raises the DC signal level, or transmits Manchester 1 bits). The signal is disabled (carrier attenuated at least 3×, DC signal level lowered, or Manchester 0 bits transmitted), at one of three times during the bit interval:

• After 0.2 of a bit time, to encode a binary 0
• After 0.5 of a bit time, to encode a binary 1
• After 0.8 of a bit time, to encode a marker bit

Bit 0 is the frame marker bit P_r. Every 10th bit starting with bit 9, 19, 29, ... 99 is also a marker bit, known as position identifiers P₁, P₂, ..., P₉, P₀. Thus, two marker bits in a row (P₀ followed by P_r) marks the beginning of a frame. The frame encodes the time of the leading edge of the frame marker bit.

All other bits are data bits, which are transmitted as binary 0 if they have no other assigned purpose.

Generally, groups of 4 bits are used to encode BCD digits. Bits are assigned little-endian within fields.

• Bits 1–4 encode seconds, and bits 6–8 encode tens of seconds (0–59)
• Bits 10–13 encode minutes, and bits 15–17 encode tens of minutes (0–59)
• Bits 20–23 encode hours, and bits 25–26 encode tens of hours (0–23)
• Bits 30-33 encode day of year, 35-38 encode tens of days, and bits 40–41 encode hundreds of days (1–366)
• Bits 45–48 encode tenths of seconds (0–9)
• Bits 50–53 encode years, and bits 55–58 encode tens of years (0–99)
• Bits 80–88 and 90–97 encode "straight binary seconds" since 00:00 on the current day (0–86399, not BCD)

In IRIG G, bits 50–53 encode hundredths of seconds, and the years are encoded in bits 60–68.

Not all formats include all fields. Obviously those formats with 60-bit frames omit the straight binary seconds fields, and digits representing divisions less than one frame time (everything below hours, in the case of IRIG D) are always transmitted as 0.

No parity or check bits are included. Error detection can be achieved by comparing consecutive frames to see if they encode consecutive timestamps.

Unassigned 9-bit fields between consecutive marker bits are available for user-defined "control functions". For example, the IEEE 1344 standard defines functions for bits 60–75.