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Appendix B IRIG Protocol Overview
For example, IRIG-B 120 indicates that the information is sent once per second, 100 bits per
second, on a 1kHz amplitude modulated sine wave, and that the information sent is the time of
year in BCD, control bits, and the seconds of day in straight binary seconds.
There are 3 types of “bits” sent in the IRIG standard: binary zeroes, binary ones and position
identifiers. To transmit a binary zero, the source must keep the signal at mark for 20% of the bit
duration and at space for the remaining 80%; to transmit a binary one the source must keep the
signal at mark for 50% of the bit duration and at space for the remaining 50%; to transmit a
position identifier, the source must keep the signal at mark for 80% of the bit duration, and at
space for the remaining 20%. Binary bits are used to transmit information such as time of year,
straight binary seconds, and so on, and position identifiers are used to separate the different
pieces of data transmitted. The second’s boundary is embedded into the transmission by sending
two consecutive position identifiers; the beginning of the second position identifier is the
second’s boundary.
For pulse width modulated systems, conventional digital binary signaling is used, and mark is
defined as the logic high state, while space is defined as the logic low state.
Table B-2. IRIG Option Characteristics
Modulation type
Carrier Signal
Frequency
Information sent
0 Pulse width
modulated
0 DC 0 TOY (BCD), CB, SBS
1 Amplitude
modulated (sine
wave)
1 100 Hz 1 TOY (BCD), CB
2 Manchester
modulated
2 1 kHz 2 TOY (BCD)
3 10 kHz 3 TOY (BCD), SBS
4 100 kHz 4 TOY (BCD), Year (BCD),
CB, SBS
5 1 MHz 5 TOY (BCD), Year (BCD),
CB
6 TOY (BCD), Year (BCD)
7 TOY (BCD), Year (BCD),
SBS
CB = control bits
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