So they had an Ethernet from the module to each graph and then the other Ethernet port back to the other graph? I guess that certainly could cause a screwed up situation, makes sense now.

Actually, with ANY Structure-Scan Module (not just 3D-SS) the primary Internet cable connections should be from the SS module to each sonar/graph unit. The Lowrance SS modules all have an Ethernet router built in, typically 3 port in the case of Lowrance. When you connect each unit back to the SS built-in router, you're building a network that allows any two units to communicate. So, you actually DO still have Ethernet from unit to unit, but it is no longer implemented with a cable directly between the two units.

The router executes a "discovery" protocol when the system is powered up. This is what lets it discover what devices are connected to the network so that they can assigned ports and allowed to communicate correctly. If you keep a previously installed unit-to-unit Ethernet connection in place, that makes a redundant connection which is not reconcilable by the router's discovery process. All bets are off then.

HDS Gen 2 and Gen 3 units have two-port routers built in. This is why a simple network is possible without a SS module. When a SS module is added, the old network wiring must be disconnected, then the connections rewired using the SS module as the network hub. The installer that left the pre-existing wiring in place (creating a redundant connection) didn't know what he was doing.....and shouldn't be paid like he did.

Yes. I had to remove each end of the Ethernet cable from both flush mounted units.

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