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I suspect that the depth is dropping out because it is genuinely not getting a reflection off the bottom. This could be because you are in very deep water (but it is unlikely that your slip is more than 200ft deep ;) ) or, more likely, because you have marine growth around, or even inside, the through-hull.

I had a problem with speed always reading zero on my DST800, and it was marine growth jamming the paddle. I now have a round foam dish scrubbing brush, bought from the supermarket, that fits nicely down the through-hull. I give it a scrub now and then.

When was your bottom last cleaned? Even if it was cleaned recently, they can't reach inside the
through-hull.

Word of warning for later when you install the wind instrument : they normally have a terminating resistor, so get installed on the END of the network (in place of one of the terminators) - NOT as a drop.
 

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I don't see why that would cause problems - it is fine to feed NMEA 2000 with power in more than one location (as long as the polarity is the same!) to overcome voltage drops in long cable runs.

Edit : It isn't fine. See below.
 

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Oh I see, I think it's to do with noise loops.

Well some of the MFDs make a big thing of the NMEA 0183/2000 repeater function, so there must be a way to do it right (meaning they take 0183 data in, and output it in 2000 form)
 

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http://static.garmincdn.com/pumac/GMI_10_INST_EN.pdf

http://static.garmincdn.com/pumac/GMI10_InstallationInstructions-S_N19wxxxxxx.pdf

The plot thickens, because on the Garmin website there are two versions of the instructions. If you look at the diagrams on pages 5 / 6, the thing that has changed is, (guess what!) in the newer version the + supply is no longer connected to the GMI 10 via the NMEA 0183 connector - only to the GPS sensor.

The two manuals reflect a change to later versions of the GMI 10, which get their power via NMEA 2000. It seems that if the OP has the newer version, he is using the old, wrong, instructions.
 

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Hi everyone and thank you for your input.

Some info: The boat is on the hard and the depth sounder gave readings at some point when pointed either to the ground below or to thick GRP sections of the boat itself (incorrect depths to be exact - probably because it gets readings through the air and not water)

In the manual I used it says that GMI 10 DOES NOT get power through the NMEA2000 network. In fact, I tried both ways, and GMI 10 DID turn on when only powered by the NMEA2000 bus. In either case the results were the same.
I will give the common ground thing a go when I get back to the boat, since it seems the only thing that makes some sense to affect normal operation by cancelling noise on the data lines and the one thing I haven't tried.

Again, thank you all for chiming in. :)
It appears that versions of the GMI 10 since 2008 have been powered by NMEA 2000. So either you bought some very old stock, or that's the wrong manual for yours.

I don't think depth transducers work reliably in air. Try it in a bucket of water?
 

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I'm not convinced that there is a problem, at least with the depth transducer. They are not intended to work in air - if it ever did, that was a fluke.

It DOES sound like there is something funky about the NMEA 0183 wiring, but that might be fixed by re-wiring to the latest wiring diagram.
 
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