ComNav Commander P2 & P2VS Installation & Operation Setting Up
Document PN 29010074 V4.1 - 96 -
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However, since the Baud Rate of the SPU’s NAV2 input can be either 4800 or
9600
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, the max data rate is 960 bytes/sec … and so both HDT (or HDM) & HDG
can
be used into NAV2, at 10 Hz each.
This check must include any other NMEA sentences output from the NMEA compass.
As an example: the Vector G2/G2B can also output the boat’s Speed Over Ground (in the
VTG sentence – 45 bytes), which can then be used as the autopilot’s Speed Source data
input:
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The factory default setting of the G2/G2B is HDT & HDM at 10 Hz each, and VTG at
1 Hz (on Port B). That is indeed useable on NAV1, as well as on NAV2:
–
20 x 10 + 20 x 10 + 1 x 45 = 445 bytes/sec … < 480 bytes/sec
The total bytes/sec of all sentences must always be less than the link’s max
data rate!
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 discards them.
But that causes subtle problems. For example, if the Heading sentences are
being truncated, a few will usually still get through OK, and so the SPU will not
raise a NO HEADING alarm (the SPU’s error-detection routines do not measure
update rate, but only check if
any
valid data is being received). However, the
effective Heading update rate will be less than is required for “good 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 are getting though OK per second, then
the effective Heading update rate that the autopilot is using is 2 Hz … and that’s
much, much
too low for “good 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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NAV2’s Baud Rate is automatically set by the SPU – by looking for “correct” NMEA sentences at each rate possible.
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