In most telemetry applications, measurand values change at different
rates, often by several orders of magnitude. There is no need to sample
slowly changing measurands as frequently as quickly changing measurands.
The slowest changing data may not even require sampling once per frame.
The concept of a major frame was therefore developed to include multiple
frames, each called a minor frame.
Multiple slow changing measurands can share a single frame word (word
4 in the above illustration). This slower sampling rate is called subcommutation.
To distinguish between the meaning of this shared position between minor
frames, a subframe synchronization scheme is required. The value of the
contents of another word in the frame is assigned the task of identifying
the current minor frame. Details of subframe synchronization appear later.
The figure below illustrates subcommutation with the symbolic representation
of four sub-commutated channels, which share the sixth channel of the
main commutator frame. The makeup of each frame is different.
It takes five revolutions of the main commutator to sample every sensor
at least once. These five frames together are called the major frame.
Each pass of the main commutator produces a minor frame. The wheel shown here is a rather simplistic example. In a typical operation,
it is not uncommon to use 64 minor frames per major frame, with 512 words
per minor frame. The size is not to accommodate a large number of different
measurands, but to satisfy a large disparity of sampling rates (e.g.,
temperature versus an accelerometer).
Paragraph 4.3.2 of the IRIG-106 Standard illustrates the major frame
as a two-dimensional matrix with the minor frame as one row.