ComNav P4 - Bandwith of a NMEA Compass Connection

ComNav P4 Installation and Operation Getting Started
Document PN 29010100 V1r0 - 110 -
Be advised that some NMEA0183 compasses, even if correctly wired, will sometimes be
outputting Null Heading values. For example, the G2/G2B, if it can not detect enough GPS
satellites well enough to be able to compute the Heading, it will output these sentences:
 $GPHDM,,M*1B
 $GPHDT,,T*1B
In these cases, the Heading data fields (between the commas) are empty, meaning “null”
or “nothing” (not “zero” – since 0 is a valid Heading value). This will result in no Heading
being displayed on the Control Head, and trigger a NO HEADING alarm. To deal with this
condition, please refer to the NMEA compass’ manual – and in the meantime, use one of
your other on-board compasses.
Bandwidth of an NMEA Compass Connection
You must always ensure that the maximum bytes/second capability (aka “data rate” or
“bandwidth”) of the serial link between the NMEA compass and the autopilot is not
exceeded by the enabled sentences.
The maximum data rate of an NMEA 0183 serial link is always 1/10th of the link’s Baud
Rate. An example calculation:
 The Baud Rate of the SPU’s NAV1 input is fixed at 4800, so if the NMEA
compass is connected to NAV1, it must be configured to output at 4800
Baud, and the max data rate is then 480 bytes/sec.
 HDT and HDM sentences are each 20 bytes long, and HDG is 33.
Thus, it is possible to have both HDT and HDM at 10 Hz each, connected into NAV1:
– 20 x 10 + 20 x 10 = 400 bytes/sec … < 480 bytes/sec
But it is not possible to have both HDT (or HDM) and HDG into NAV1:
– 20 x 10 + 33 x 10 = 530 bytes/sec … > 480 bytes/sec
However, since the Baud Rate of the SPU’s NAV2 input can be either 4800 or 9600, the
max data rate is 960 bytes/sec. So both HDT (or HDM) and HDG can be used into NAV2,
at 10 Hz each (NAV2’s Baud Rate is automatically set by the SPU – by looking for “correct”
NMEA sentences at each rate possible). This check must include any other NMEA
sentences output from the NMEA compass.
As an example: the 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:
The factory default setting of the G2/G2B is HDT and HDM is at 10 Hz each, and VTG is
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.