One or
more completely independent, slower, asynchronous data sources, including
those from an avionics subsystem, a serial output, a computer,
or a video sensor, can be merged with the main telemetry stream to form
an embedded asynchronous data stream (EADS). These embedded streams
include
overhead and synchronization words and are inserted into reserved words
in the main frame. The embedded asynchronous data stream is a completely
independent major frame and includes frame and subframe sync words.
Each
EADS word can be a discrete binary, octal, decimal, or hexadecimal format.
CRC words can also be sent in the minor frame word along with flags
for
stale data (new data not present when required by the encoder) and overflow
data (more data available than allowed for in the main frame). Since
the
EADS’ embedded frame data rate is asynchronous to the minor frame, there
is a chance that either more data is available than can fit in the EADS,
with resultant data loss, or that there is insufficient data available
to fill the frame.
The term asynchronous is used
with respect to the appearance of the EADS’ sync words. The EADS frame
sync moves in time among the EADS words. This is the case in a frame
with multiple EADS word locations where frame lengths
are not even multiples of each other. The main frame may also be designed
such that the main encoder is either in word synchrony with the EADS
stream
or that filler words are added when real data is not yet present (the
ground system will need to remove this extraneous data).
At the ground station, a hardware
decommutator extracts and directs the EADS words to an independent
hardware decom for decommutation. Multiple
independent EADSs can be supported by one main stream. Each of the streams
is sent to its own decommutator. In theory, each EADS could have its
own
EADS, but in reality, this low sample rate is not seen. If the EADS is
a much slower stream than the main stream, decommutation can be accomplished
by a "software decom" in a front-end real-time processor
or workstation. The IRIG-106 Standard indicates a maximum of two independent
EADSs per major stream.
The example below requires transmission of 11 major frames to receive
two EADS frames (assuming that the word lengths of both frames are identical).
This is not a constraint for the EADS word in the main frame; it is just
a series of bits in the embedded stream.
