CaptTom — it’s just taken me a while to come up with a good example for you. When I initially outfitted this
boat, I had enough
money, time and access to dealer-prices from a friend in the business. So I bought all the latest-greatest stuff.
The fanciest wind-speed-depth (DST) system. Latest
radar. I wanted the radar/plotter to be able to overlay chart and
radar data. That meant high-speed heading data from the
autopilot, an extra cost option. All installed, all working just fine, except that every rare once in a while the DST system would lock up. About every week to ten days if you left it running. Hmmmmm, but not a big problem.
Next, hook up the
autopilot 10Hz compass output to the radar for overlay. Works beautifully.
Then I decide to hook up the compass data to the DST to enable display of True Wind Speed/Direction and
cockpit display of heading data. Works fine but now the DST locks up about every 3 days. That’s not getting better.
So then somebody suggests that I connect the
GPS to the autopilot so that I can display it in the cockpit and steer to a waypoint. Works fine, except that now the DST system locks up in minutes, not days.
After a lot of
head scratching, and a million calls to the tech support people, and on-boat visits by two factory people, we found the problem.
If the DST system was given "too much data over the
network," it would lock up. Nobody knew quite how much was too much. The autopilot was putting out magnetic heading data at 10Hz. But it also was putting out unused and unneeded
Rudder Position data every time it put out heading. So now the network load is doubled, causing the quicker failure of the DST.
What did the
GPS have to do with this? It turned out that the GPS not only outputted position data, but one of the previously unused fields, Magnetic Variation, was now included. When the autopilot saw variation data, it decided to put out True Heading data as well as magnetic. That meant the network traffic was now triple what it had started at, which was enough to push the DST system over the edge.
The DST vendor sneaked around the problem by giving me a very expensive slower compass that hooked up directly, thus avoiding the problem.
So everything was working precisely as it was supposed to, but it wouldn’t work together.