IRIG timecode

Inter-range instrumentation group timecodes, commonly known as IRIG timecode, are standard formats for transferring timing information. Atomic frequency standards and GPS receivers designed for precision timing are often equipped with an IRIG output. The standards were created by the Tele Communications Working Group of the U.S. military's Inter-Range Instrumentation Group (IRIG), the standards body of the Range Commanders Council. Work on these standards started in October 1956, and the original standards were accepted in 1960.
The original formats were described in IRIG Document 104-60, later revised and reissued in August 1970 as IRIG Document 104-70, upgraded later that year as the IRIG Document to the status of a Standard, IRIG Standard 200-70. The latest version of the Standard is IRIG Standard 200-16 from August 2016.
Timecodes.
The different timecodes defined in the Standard have alphabetic designations. A, B, D, E, G, and H are the standards currently defined by IRIG Standard 200-04.
C was in the original specification, but was replaced by H.
The main difference between codes is their rate, which varies between one pulse per minute and 10,000 pulses per second.
IRIG Code B Calculation:<br>
Bit rate = 100 Hz = 100 × (1 / second) = 100 / second = 100 / 1000 ms = 1 / 10ms<br>
Bit time = 1 / (Bit rate) = 1 / (1 / 10 ms) = 10 ms<br>
There are 100 Bits per frame.<br>
Frame time = (Bits per frame) × (Bit time) = 100 × 10 ms = 1000 ms = 1 second<br>
Frame rate = 1 / (Frame time) = 1 / 1 second = 1 Hz<br>
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The bits are modulated on a carrier. A three-digit suffix specifies the type and frequency of the carrier, and which optional information is included:
Binary-coded decimal (BCD) day of year, hours, minutes, and (for some formats) seconds and fractions are always included. Optional components are:
The recognized signal identification numbers for each format according to the standard 200-04 consist of:
Thus the complete signal identification number consists of one letter and three digits. For example, the signal designated as B122 is deciphered as follows: Format B, Sine wave (amplitude modulated), 1 kHz carrier, and Coded expressions BCDTOY.
The most commonly used of the standards is IRIG B, then IRIG A, then probably IRIG G. Timecode formats directly derived from IRIG H are used by NIST radio stations WWV, WWVH and WWVB.
For example, one of the most common formats, IRIG B122:
Timecode structure.
IRIG timecode 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 timecode 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:
Bit 0 is the frame marker bit Pr. 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 Pr) 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.
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.
IRIG J timecode.
IRIG standard 212-00 defines a different time-code, based on RS-232-style asynchronous serial communication.
The timecode consists of ASCII characters, each transmitted as 10 bits:
The on-time marker is the leading edge of the first start bit.
IRIG J-1 timecode consists of 15 characters (150 bit times), sent once per second at a baud rate of 300 or greater:
 <SOH>DDD:HH:MM:SS<CR><LF>
At the end of the timecode, the serial line is idle until the start of the next code. There is no idle time between other characters.
IRIG J-2 timecode consists of 17 characters (170 bit times), sent 10 times per second at a baud rate of 2400 or greater:
 <SOH>DDD:HH:MM:SS.S<CR><LF>
This is the same, except that tenths of seconds are included.
The full-timecode specification is of the form "IRIG J-"xy"", where "x" denotes the variant, and "y" denotes a baud rate of 75×2"y".
Normally used combinations are J-12 through J-14 (300, 600, and 1200 baud), and J-25 through J-29 (2400 through 38400 baud).