ComNav Commander P2 - Bandwidth of an NMEA Compass Connection

ComNav Commander P2 & P2VS Installation & Operation Setting Up
Document PN 29010074 V4.1 - 95 -
2) Whenever an NMEA compass is being used as the Heading source (see page 117), if
valid Heading/Compass data sentences are not received for ~3 seconds, an INVALID
HEADING alarm will occur.
3) If you are going to be using only Magnetic Heading values, or only True, then you only
need to set the NMEA compass to output the corresponding sentence – HDG or HDM for
Magnetic, HDT for True.
4) If you wish to be able to switch between True & Magnetic (via the NMEA Heading Type
parameter in the Standby menu – see page 118), then HDT and either HDG or HDM
should be enabled, with both HDT & HDG/HDM at 10 Hz (but see the section on
“Bandwidth”, below).
To verify that an NMEA compass is connected, look at the NMEA IN LEDs in the Diagnostic
section; the LED for the SPU’s NAV input which the compass is connected to should be
flickering.
Note: if the `A´ & `B´ wires are connected backwards, the LED will still flicker; but
when NAV1 or 2 is later selected at the Compass Source in the Standby menu,
no Heading will be displayed, and a NO HEADING alarm will occur. If this is the
case, simply swap the `A´ & `B´ wires.
It’s worth noting that some NMEA compasses, even if correctly wired, will
sometimes be outputting Null Heading values. For example, the Vector G2/G2B,
if it can not “see” enough GPS satellites well enough to be able to compute
Heading, will output these sentences:
$GPHDM,,M*1B
$GPHDT,,T*1B
In this case, the Heading data fields (between the commas) are empty, meaning
“null” or “nothing” (not “zero” – since that 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 be sure that maximum bytes/second capability (aka “data rate” or
“bandwidth”) of the serial link between the NMEA compass & the autopilot is not exceeded by
the enabled sentences.
The maximum data rate of an NMEA 0183 serial link is always 1/10
th
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 & HDM sentences are each 20 bytes long, and HDG is 33.
• Thus, it is possible to have both HDT & 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) & HDG into NAV1:
– 20 x 10 + 33 x 10 = 530 bytes/sec … > 480 bytes/sec