GARMIN 188c or 298 to VHF UNIDEN UM625c for DSC NMEA Operation

[Rasputin]

Greetings,

I have both a GARMIN 188C and 298 GPS Charplotter and UNIDEN 625c down in my chart room. Both GARMIN units are the EXACT same wiring harness and diagram in their respective manuals and confirmed by GARMIN Support. I've unsuccessfully been trying to connect either to my Uniden 625C VHF for DCS operation. I have the GPS unit set at NMEA Serial 4800 Baud but have also tried the "Garmin Data Transfer" setting. I've called GARMIN and UNIDEN support multiple times and am sure the wiring is correct.

I would post PDF links to both manuals w diagrams but I’m unable to do that with my access as of yet.

I’ve tried everything to get the UNIDEN to see the GARMIN with no luck. I know I have the wiring right. What gives? What am I missing?

From the manuals:

UNIDEN 625c VHF:
Orange – NMEA 0183 Out (-)
Yellow – NMEA 0183 In (-)
Green – NMEA 0183 Out (+)
White – NMEA 0183 In (+)
Red – Not used
Brown – Speaker
Blue – Speaker
Black – Not used.

GARMIN 188c & 298:
Red + VDC
Black Ground
Orange Accessory On
Blue NMEA Out
Brown NMEA In
Green C A Net L
White C A Net H
Yellow Alarm Low

My UNIDEN to GARMIN config:
White NMEA In (+) to Blue NMEA Out
Green NMEA Out (+) to Brown NMEA In
Orange & Yellow NMEA In/Out (-) to Black Ground

Butkis. I’m sure I’m missing something simple. I’ve looked at a number of posts on other blogs and am convinced I’m jinxed.

Thoughts? Ideas? What am I screwing up?

Thanks!

~Scott

[MarkSF]

I had some issues with my SH GX2150 VHF, and the Lowrance HDS-5m plotter. Now this might be more relevant than you think at first glance, as the Lowrance has singled-ended connections, while the GX2150 VHF has double-ended, just like your setup.

The - connections at the SH are inverted versions of the + output. They are not intended to be grounded. They should either be left floating, or tied to ground with a resistor.

So I think your cable arrangement should be :

White NMEA In (+) to Blue NMEA Out
Green NMEA Out (+) to Brown NMEA In
Orange & Yellow NMEA In/Out (-) unconnected
VHF ground to Garmin ground

In the case of the latter, there should either be a shield on the VHF's output cable, or you'll have to use a chassis ground.

I too found a lot of misinformation on the interweb, including from Lowrance support, but in the end asked a PhD electronics engineer at work, how to connect single ended to double ended. He told me to do it like above, and it worked straight away.

Right now it's not working because those - connections aren't ground. Therefore, the circuit is not complete. You need a complete circuit for current to flow / data to be transferred, and for that you need to complete the ground.

[Rasputin]

I'll give it a whirl this weekend. Thanks!

[redline]

Originally Posted by MarkSF

White NMEA In (+) to Blue NMEA Out
Green NMEA Out (+) to Brown NMEA In

Agreed, but depending on exactly how Uniden is driving their NMEA outputs, you may also need to connect:

Orange & Yellow NMEA In/Out (-) to Garmin ground

Try that if it doesn't work with Mark's suggestion.

[MarkSF]

If those are inverted outputs there is no benefit to grounding them, indeed you could blow the buffer ICs. They should be left floating.

I guess the question is, are these true differential outputs, or two outputs, one inverted and one not? On the GX2150, they are the latter. On the Uniden, I don't know, and the manual is no help (I just looked).

[Rasputin]

Is that something worth me calling Uniden and asking?

[MarkSF]

Honestly, I think your plan to try it wired the other way this weekend was the way to go!

[redline]

The Uniden "NMEA In -" should be safe to ground, and if they designed the radio really nicely, its "IN +" and "IN -" would drive an optocoupler with no other ground reference, so you would *have* to connect "In -" to GPS ground to get it to work. I can't see how you could damage anything by grounding the "In -", and it's easy to check - if it is indeed fully isolated, connecting only "In +" won't be sufficient. I'd suggest you try the GPS-out to VHF-in path first, without worrying about the other direction.

For the radio's "NMEA out", it would be great if you could have someone send you a DSC Individual Call whiile you have a voltmeter on the "out" leads... The "Out +" should flicker between 0 (measured to ground) and somewhere between 5 to 12. If not, measure from "Out +" to "Out -" instead (in the unlikely case that they went to the trouble of generating an isolated output voltage source - if so I'd be very impressed). You could also measure the resistance between "Out -" and "Ground" (in both "directions" - swap the ohmmeter probes around in case there's a diode in the circuit), which doesn't require that someone send you a call.

Let us know what works. I'm betting the "VHF In - to GPS ground" will do the trick for GPS->VHF.

[MarkSF]

He already tried that, and it didn't work. See the first post, and the description of his current wiring diagram. That's why I'm suggesting the alternative arrangement, which is what SH recommend for their VHF with non-inverted and inverted inputs and outputs.

See below, for the GX2150, connecting Lowrance HDS series to the VHF. They have left the inverted inputs/outputs floating. This is the wiring arrangement I've been using for 18 months, which worked, after trying it with the - conns grounded, which did not.

Welcome to StandardHorizon.com

[redline]

Another possibility is a documentation error. I've owned at least one piece of equipment whose manual had a single-sheet insert stating that the colours / wire assignments were reversed in the manual. I've even debugged an installation in which the chartplotter had a series resistor and a diode to ground (basically a polarity/overvoltage protection circuit) on its OUTPUT lead. I could understand that on the input, in cae of PC or 12VDC connection, but on the output it reduced the signal level to the point where his radio saw nothing. Removing the resistor got everything working 100%. Good thing I'd brought an oscilloscope with me that day - would not have been evident on any regular voltmeter. Meant to write the manufacturer asking "why" - but it could even have been a manufacturing error on the cable.

The moral of the story is - you never know what to expect.

p.s. since the above, I've fallen in love with my Oscium - iMSO-104 Mixed Signal Oscilloscope - a neat and tiny gadget that plugs into your iPhone or iPad. Wish I'd had that back when I was routinely lugging a CRT oscilloscope around the field.