[CA] [CA11] Axiom Pro RVX Course Up Issues.

[Chuck - Raymarine - Moderator]

Matt,

While I understand your desire to remove possible sources of the problem reported, the DSM would have no bearing and as such need not be disconnected from the system. Per our discussion, both the Axiom Pro MFD and Axiom MFD are reporting HDOP values of 0.8 when operated independently. The resulting GPS performance should be identical. However, as you have indicated, the COG is significantly greater. Per our conversation ... and not indicated within your diagram, the Axiom Pro MFD has been interfaced to a VHF radio throughout this investigation, which is by your recollection, an Icom VHF radio. As indicated, we have had some past experience with Icom VHF radios looping received NMEA 0183 data. The problem with such is that if the MFD has been interfaced in a manner which would support bidirectional communications with a device which is looping NMEA 0183, the MFD will perceive it to be an additional NMEA 0183 source of GPS data ... and by LH3 prioritization schema, assign it highest priority. This may have the added effect of causing the apparent doubling of the local magnetic variation applied to COG. In the Seattle area, the local magnetic variation is approximately 15.5 degrees. The suggested data loop may result in approximately 31 degrees of local magnetic variation being applied instead of 15.5 degrees. Accordingly, I suggest that the BAUD rate setting of the Axiom Pro MFD's NMEA 0183 Port which has been interfaced to the VHF radio be changed from a matching BAUD rate (typically 4800 for DSC VHF radios lacking AIS features) to a mismatched BAUD rate (i.e. 38400), thereby breaking communications between the MFD and VHF radio, and that the system then be tested again. I further recommend that the Mercury Marine devices be disconnected during this test. Should the system perform correctly after having made the specified changes, then BAUD rate of the Axiom Pro MFD's NMEA 0183 Port may be changed back to its original setting and the system tested ... I suspect that you may find that the symptom returns. If not, then the Mercury Marine devices may be reconnected and tested one by one to determine whether either causes the reported issue.

Should the problem be determined to be solely attributed to the VHF radio, then the following two options would apply:
a) disconnect the MFD's NMEA 0183 input leads from the VHF radio's NMEA 0183 output leads; or
b) install a programmable NMEA 0183 multiplexer (ex. Actisense NDC-4-USB, etc.) between the MFD's NMEA 0183 input leads from the VHF radio's NMEA 0183 output leads. The NMEA 0183 multiplexer would then be programmed to only pass the DSC and DSE NMEA 0183 sentences to the MFD.