Two NMEA inputs into DSC VHF

[[3889]]

I have an oldish GPS unit supplying NMEA data to my VHF which normally serves no other purpose. I would like to change the primary NMEA source to my plotter but also keep the existing system as a backup. If both GPS sources were to inadvertently transmit presumably slightly different NMEA data simultaneously to the radio is this likely to cause lasting damage?

 

[Cardo]

It should simply confuse it slightly.

I have a similar setup, although I keep the backup GPS turned off, and would only turn this on if the primary GPS output fails.

 

[nigelmercier]

If you are using it as backup, fit a switch or plug and socket.

 

[lustyd]

I have an oldish GPS unit supplying NMEA data to my VHF which normally serves no other purpose. I would like to change the primary NMEA source to my plotter but also keep the existing system as a backup. If both GPS sources were to inadvertently transmit presumably slightly different NMEA data simultaneously to the radio is this likely to cause lasting damage?

No it would just flip flop between positions and potentially cause some info on the wire to be lost in the shouting match between systems. A switch will sort it, if one fails then manually change. The radio will scream when it loses fix so no worries about not noticing.

 

[[3889]]

Yes, a switch. Obvious really

Thanks.

 

[nigelmercier]

Yes, a switch...

A DPDT.

 

[Boo2]

If you are using it as backup, fit a switch or plug and socket.

I also have a similar need, do you know of any suitable switches, Nigel ? I would prefer to be able to switch between 4 sources : New e7 chartplotter (default), Handheld (emergency - battery powered) GPS, Handheld VHF and existing (v. old) Philips Mk9 GPS. Some kind of rotary select swich would be ideal but I don't know of any dual pole 4-position switches suitable for marine use ?

Boo2

 

[David2452]

No, the 0183 data is at best unreliable when you connect two together, take each of the + 0183 data leads to the load side of an on/off/on switch and take a single data lead from the switch terminal that you would normally connect to power. make sure the switch is in a protected area so it can not inadvertantly be operated.

 

[Norman_E]

NMEA 0183 is a single talker multiple listener system so having two talkers on the same circuit is at best inadvisable. If you are connecting a plotter to a DSC VHF then you probably need to connect NMEA out from Plotter to NMEA in on the radio, to send the GPS data (sentence GGA ) to the VHF and connect NMEA out from the VHF to NMEA in on the plotter so that the VHF can send DSC polling data to the plotter, including sending the received position from a DSC emergency message. Any switch used to change over between using the plotter to send GPS position information and the backup GPS should be a double pole changeover switch in the first of the above circuits. The issue of connection between VHF and chart plotter is confused by the different terminology used by manufacturers and by the different connection systems used in NMEA 0183. See page 9 of the Actisense NMEA 0183 information sheet. Download it from here http://www.actisense.com/products/nmea-0183/nbf-3/downloads-nbf-3.html third item down.

 

[Burnham Bob]

maplinsused to do a rotary switch 'kit' where you simply made up the number of outputs by adding discs and soldering connections but these dyas you can just buy a rotary switch off the shelf that will cope with four different circuits