There is a good presentation from Lowrance that originated in the dawn of the HDS series that explains what data is carried between devices on what network connection. The presentation is actually a series of slides in a PowerPoint presentation, and it is called HDS SYSTEM OVERVIEW. The file used to be hosted by Lowrance at their main website host, but it is no longer there. You can find it at other hosting sites. Here is a link:
http://lowranceprostaff.dk/download/low ... erview.pdfIf you read this document, you will find a chart showing a matrix of data and networks. It will have a line for AIS and indicate that AIS is shared over NMEA-2000 networks. But, as I mentioned earlier, that is really a bit misleading because AIS data does not originate in the HDS. If there were HDS data on a NMEA-2000 network, then, of course, that data would be available to other HDS devices on the NMEA-2000 network as well as to the first HDS device.
The chart does mention two categories, "Data source settings" and "System settings" as being shared over Ethernet. You might try an experiment. Disconnect one of the HDS devices from the Ethernet. Then change the settings on the other device for its serial ports. Then check to see if the HDS that was not on Ethernet had its settings for the serial port changed. This would tend to confirm that the change in serial port settings was migrating via Ethernet.