NMEA Comms Question

[Major_Clanger]

I've just plumbed-in a Garmin 451 plotter and am using the 0183 output for a NASA GPS repeater and Icom M423 VHF. The repeater works fine but the radio isn't receiving any info as showing 'no position'. I've double-checked that I'm using the radio's positive 'listening' wire. Any ideas what I've done wrong?

Thanks for any input.....

 

[andsarkit]

You probably need to connect both 'listener' wires.
Green = B, yellow = A. It is a differential connection not referenced to ground. If one of the wires has been grounded it may possibly work as the NMEA implementation is sometimes loosely interpreted but it is not the proper way to do it.

 

[andsarkit]

I think both the Garmin and Nasa use a single ended connection with NMEA ground connected to 0V. This is possibly not compatible with your Icom differential input.
Some more information here: https://www.navcen.uscg.gov/pdf/gmdss/taskForce/NMEA_7.pdf

 

[MikeCC]

Usually just connecting ICOM signal -ve to Garmin ground will do the job. There's not really any practical alternative.

 

[Major_Clanger]

I think both the Garmin and Nasa use a single ended connection with NMEA ground connected to 0V. This is possibly not compatible with your Icom differential input.
Some more information here: https://www.navcen.uscg.gov/pdf/gmdss/taskForce/NMEA_7.pdf

Thanks, that's a really useful link.

 

[PaulRainbow]

Usually just connecting ICOM signal -ve to Garmin ground will do the job. There's not really any practical alternative.

I agree with that Mike. In fact, in most cases it works if you just connect the Icom signal neg to its own -12v power cable. Neither method is how it was supposed to be done, but as you say, no choice.

 

[Major_Clanger]

Thanks chaps, all sorted. I had thought I could use one talker for two listeners and hadn't realised I had a second talker port. Connected the VHF to that and all is fine.

 

[pvb]

I had thought I could use one talker for two listeners...

You can.