Describes the IRIG-B timing protocol.
IRIG-B is one of six serial protocols for distributing time codes currently defined by the Inter Range Instrumentation Group (IRIG). The six IRIG protocols mainly differ in the frame size or the amount of time before new information is sent.
IRIG-B is the most common IRIG standard and the one NI-Sync devices use. IRIG-B is an encoded TTL signal that carries the absolute time. It is similar to a pulse per second (PPS) signal, but instead of outputting a single uniform pulse every second, IRIG-B sends coded bits that make up a one-second-long data frame, and it repeats or re-synchronizes every second. IRIG-B sends coded bits that make up a one-second-long data frame, and it repeats or re-synchronizes every second. IRIG-B specifies that a 100-bit time frame is transmitted once per second, with each bit being represented as a 10 ms period.
The 0 bit represents 2 ms of a high logic state, the 1 bit represents a 5 ms high, and the P bit represents an 8 ms high. The P bit also separates seconds form minutes, minutes from hours, and so on, within the one-second frame. Data in the time frame includes Binary Coded Decimal (BCD) time of year, year, and straight binary seconds (SBS). The data can be DC biased (DC) or amplitude modulated (AM) with a 1 kHz sine wave.
The reception of the first bit of an IRIG-B data frame generates a timestamp for the event. You cannot read or use the timestamp as a time reference until the entire IRIG-B frame has been received and decoded. After the frame is successfully decoded, the frame drives the time reference engine if you have set IRIG-B as the external time reference. Both the timestamp generated by receiving the first frame bit and the time/date encoded in the IRIG-B frame can be read using Read Last GPS or IRIG Timestamp.