ComNav P4 Installation and Operation Getting Started
Document PN 29010100 V1r0 - 111 -
Problems caused by exceeding the max data rate are usually very hard to
troubleshoot. The
excessive rate will cause data errors, mostly in the form of partial sentences
(caused by
buffer memory overflow in the sending device, and/or in the SPU). These are always invalid,
so the SPU simply discards them.
However that also can cause other subtle problems. For example, if the Heading sentences
are
being truncated, a few will usually still get through, and so the SPU will not
raise a NO
HEADING alarm (the SPU’s error-detection routines do not measure
update rates, they only
check if any valid data is being received). However, the
effective Heading update rate will be
less than is required for acceptable steering – so
the autopilot will seem to be “steering
poorly.”
A specific case: if the Heading data source device is nominally transmitting valid
Heading
sentences at 10 Hz, but only 2 per second are getting though, then
the effective Heading
update rate that the autopilot is using is 2 Hz. And that’s
much too low for adequate steering.
Similar problems can occur with other NMEA data used by the autopilot (speed,
position,
waypoints, etc.), if the link max data rate is exceeded.
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