Mark.  My simnet issues brought me to this forum.  I’m new here.  I experienced some of the same problems with my system. 

I’ve read through the discussion and wasn’t sure if it was clearly mentioned, but the wind instrument IS the end of the network backbone. It has a built-in simnet terminator.  If your wind instrument fails or is not present it can bring down the network.  The other end of the Simnet Backbone is a Termination Plug inserted into a 3 way at start of the Network, at one on my 3 MFD’s

In my case the wind instrument was damaged (lightning??) and it caused an error on the network.  Nothing was being recognised.  Unplugging it helped considerably, but not completely. Installing the red termination plug in it’s place at the 3 way at the base of the mast brought everything back 100%. 

Now this weekend I notice the NAIS-300 is out.  It’s warm to the touch and it’s drawing current, but there are no indicator lights illuminated. I assume it’s toast.  Another possible lightning strike or just a failure of the unit. 

It’s been an expensive summer.  

I'm back with an update on my repairs.  

We've been in Harbour Towne Marina being repaired.  Its been 2 months.  Hopefully done in a couple of weeks.

Replacements include: communication gear, navigation gear and sensors, engine electronics, bilge pumps, watermaker, and forestay (most probable lightning path to water).  Pretty much all my 12v systems. Approx $50k - mostly covered by insurance (Pantaenius).

Negatives - dock life.  Our boating principle is anchor as much as possible.  Until the lighting strike, we were 100% anchored since May 10, 2018 our departure date and 32nd wedding anniversary.  Struck on July 11th and now we have been docked more than anchored  🙁  .

Positives - nobody hurt in the lightning event, great relationship with Just Cats folks, various system upgrades, great understanding of my boat's systems, and costs mostly covered by insurance.  Also, met 3 other boat owners impacted by lightning, and various other really nice people around the marina. And, I found this forum.

I'm now working on a connectivity system - wifi + LTE.

Thanks for all the comments and guidance.

Simnet cables fail right at the little yellow faced connector because people push too hard trying to connect them. Check all your cable ends carefully.