Nmea amplifier

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CPD

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I am not an electronics expert and get easily confused, so please go easy on me !!!

I have a garmin etrex (2 actually) one older than the other. The nmea output feeds into a DSC/VHF a radar, and also now an AIS display. The later etrex (software 3.30) does the job perfectly. The older (software 2.14) feeds the DSC but will not show on the AIS. I spoke to garmin about this and they couldnt supply too much detail but were aware it could happen. My immediate solution is quite straight forward, either have the later etrex feed all the devices, or as a backup, have it feed 2 (maybe radar and AIS) and the earlier version feeding the DSC.

However, on thinking about it, there must surely be available a very simple and very straightforward amplifier which will take the nmea signal, amplify it, and allow it to feed several devices.

I have googled "multiplexer" and come up with expensive devices that allow nmea/seatalk to be streamed together to form one nmea signal that then ............

Help !!
 
Are you sure its a voltage level problem? Maybe the AIS is expecting a different baud rate? Some bits of kit will auto sense rate, others won't.
If it is simply a matter of drive level, what you need is a buffer or driver ic or even a comparator.
 
Any GPS should be able to supply 4 listeners.

My Navman plotter would not send the correct NMEA to my older Furuno radar, but would send to the Raymarine Wheelpilot via the interface unit.

My older AP-Philips Mk6 would feed the radar and used to feed the autohelm - on our previous boat.

Obviously a slightly different sentencing configuration in the two GPS outputs, even though they were supposed to be sending the same version.

The plotter sends to the Autohelm and DSC.

The Mk6 only sends to the radar.
 
My Garmin 128 GPS happily feeds a Yeoman plotter, a C70 Raymarine plotter, a VHF radio and the AIS. There is no need for an amplifier or multiplexer. You may need to check your baud rates!
 
It isnt a baud rate issue, I checked and they are all the same. The only difference is the software version, and I tried each several times to verify it. Will play around a little more and let you know.
 
There is not normally a problem feeding multiple listening devices from a single NMEA transmitter. Garmin suggest a limit of 3, but several posters have reported success with more. I have fed 5 without problem, and am about to increase this to 6. The limiting factor is the electrical load taken by the listeners overloading the transmitter output and pulling the output voltage down, but you are unlikely to reach this point.

The only time you need a multiplexer is if you are trying to feed a single listener from several transmitters. The purpose of the multiplexer is to separate the different signals so they are not received at the same time.

In your case I suspect the issue may be the NMEA sentences. The older etrex may not be sending the particular sentence required by the AIS unit. It may be possible to switch this sentence on in the setup of the etrex.

By way of example my chartpotter (Garmin 3005) outputs position data on 3 separate sentences, GPGLL, GPRMC and GPGGA. The NASA AIS engine requires the RMC sentence, whilst the SH 3500E VHF will listen for GLL, GGA, RMC and GNS. So if I set the plottor to only output GLL (Geographic Position, Latitude and Longitude), the position would be displayed on the VHF, but not be picked up by the AIS unit.

Hope this makes sense.

John
 
If you dig around on the internet you can find software thet will display the NMEA data that is actually comming out of the GPS.

Try both GPS's and see what the difference is.

Also you can get software upgrades from Garmin' web site.

Andrew
 
Probably an NMEA version issue. your AIS is likely to be expecting a later version NMEA and the etrex is only sending the earlier version, so not enough data to get it to work.

Feed your etrex(es) into your laptop and compare the data strings.
 
[ QUOTE ]
If you dig around on the internet you can find software thet will display the NMEA data that is actually comming out of the GPS.

[/ QUOTE ]

You don't need to look that far - just use the terminal program (hyperterminal), that comes with windows.

John
 
OK:

NMEA amplifiers exist, such as this , but seem to be about £150.

You can even make your own for a tenner (pdf here )

BUT I agree with everyone else that I don't think you need one. The NMEA 0183 standard specifies that each 'talker' must be able to support several (I think 3) 'listeners' (which is what you have) and people report no problems with 5 or even 6 listeners (just parallel the connections).

The Garmin Etrex Vista software revision page suggests that they didn't output heading data over NMEA until software version 2.23, which might explain the problem with your AIS. It would appear from the same page that you can download an update for the software in your older Etrex, which might be another way of skinning the cat.

Andy
 
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