Do instruments prioritise nmea inputs?

alisdair4

alisdair4

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midnightdrifter.net
I've finally got round to connecting the NMEA output from my Lowrance HDS 7 into the rest of the boat network (so that it will feed VHF, Laptop, Radar and the tillerpilot. Up till this point, they had been fed from the GPS 152 at the nav station. I had originally planned to fit a switch to allow me to select which of the two GPS sources to use. However, when wiring the plotter in, I decided to try leaving the GPS 152 connected. With both GPS switched on, presumably the instruments are reading 2 sets of NMEA data - or doe they automatically prioritise? It doesn't seem to bother them (I had though that the small differences in, eg, position might cause some problems). If not, do I need to bother fiiting a switch?
 
You can't connect 2 NMEA outputs together into one (or more) input. It might work or it might appear to work until both outputs are sending data at the same time, then it won't work.
 
Are you using NMEA 0183 or 2000?

If 0183 then do you have separate inputs for each 'talker' into your radio etc? If not, then you will have a clash from 2 talkers (GPSs) connected together and it is unlikely to work.

If NMEA2000, there should be no problem, although you may have to configure which source you want to use for each 'listener'.
 
I'm using NMEA 0183. I presumed there would be a clash, but leaving both GPS on for several minutes seemed to cause no problem. Of course, the boat was stationary, so the VHF may have showed a stored, as opposed to a current lat and long..! Sounds like I do need to fit a switch.
 
Of course it may NOT have been working. An initial position or even an old one may have been read and used. Subsequent sentence clashes would just be ignored and/or discarded, unlikely any of your kit would have been reporting NMEA errors.
 
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