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Garmin Networking Question

Old 02-20-2015, 10:33 PM
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Iím installing a Garmin Autopilot this weekend and I was hoping that someone could clear something up for me. Iím unsure about the wording of something in the install manual. I have seen the wording before when I installed the VHF to the GPS.

ďand the black wire of the GHC data cable must be connected to the same ground as the ECUĒ

Does this mean that I need to physically attach the two wires together at a ground point, or does it simply mean that both devices need to have their grounds return to the same battery? Go easy on me, Iíve been known to over think things before.
Old 02-21-2015, 02:00 PM
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I thought it was odd too, so I just connected the two wires.
Old 02-21-2015, 07:10 PM
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Thanks for the reply, Thats what I did for the GPS to the VHF set up as well.

With the autopilot install on my boat the ECU and the battery hook up is at the back of the boat and everything else is towards the front. I will need to extend the ground wire to get it to the same ground as the ECU. My boat has a common ground so it seems to me that the ground bus at the front of the boat should be the same as the one at the back unless I'm missing some kind of magic that happens when the two wires are attached together. Hmmmm.
Old 02-22-2015, 09:43 AM
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I think the reason they do this is to equalize potentials on the neg wires. Attaching at different points on the bus could cause a difference in potential. Yes, the negative bus does carry current and it isn't always equalized.

The signals the units use to communicate are based on differential voltage. Say 2.5 v vs 5v vs 0. If one point of connection has a 1v differential then one sends 2.5 v and the other either gets 1.5 or 3.5.

You can always try it and see if it works, but I have seen it fail (on RM at least) if you didn't tie the grounds together at the same point.
Old 02-22-2015, 04:53 PM
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Hmmmm, learning all the time. Thanks for the explanation Gerg. I'm not looking for trouble so I will extend the ground to make it work the first time.
Old 02-22-2015, 07:36 PM
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Yeah, 20' of tinned 18g wife is pretty cheap. Hopefully the run won't be too hard for you.
Old 02-23-2015, 05:45 PM
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From Garmin,

Thank you for contacting Garmin International. I'll be glad to help here. The grounds must be connected to the same "Circuit" or battery.

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