NMEA - 2 GPS's

[RalphFerrand]

NMEA - 2 GPS\'s

I have already got a Garmin 128 GPS which uses NMEA to talk to a repeater, autohelm, Yeoman plotter and when switched in, the DSC unit. I have recently bought a Garmin 276c colour chart plotter, which I have mounted next to the helm. What happens if I join this into the current NMEA circuit? Does it count as a an extra burden to the NMEA chain or does it help. I am aware that one can only have three instruments connected to the GPS which is why at present the autohelm and DSC are switched in an either/or situation.
Any pointers appreciated.

 

[tome]

Re: NMEA - 2 GPS\'s

Short answer is that you will have a data collision. A GPS receiver is a Talker, and you cannot connect 2 talkers together without an NMEA multiplexer.

What you could do is connect the outputs of the 2 GPS receivers via a 2 way switch so that you could select either (eg in the event of a failure).

 

[aluijten]

Re: NMEA - 2 GPS\'s

Even with a muliplexer you would get into trouble, all the listeners would get quite confused on receiving two different GPS positions in succession. No two GPS-es will give the same position at the same time in the same place. There are 'expensive' multiplexers that can filter out certain messages to prevent the confusion.

Arno

 

[philip_stevens]

Re: NMEA - 2 GPS\'s

I've got my Navman 5500 talking to the Autohelm (via the Interface Unit to steer by Track) and the Yeoman Plotter - back-up on paper. In the coming few months, I will be fitting a Navman 7200 DSC VHF, and the 5500 will also talk to this as well.

The older AP-Philips Mk6 is talking to the Raytheon radar.

I intend fitting a new NMEA wind unit, that will be talking to both the 5500 and the Mk6. Connected to the 5500, it will allow me to "steer" the boat by wind angle, and connected to the Mk6, I will be able to see apparent or true wind angle - no reason as I don't race, it's just that the facility to see it is there on the Mk6.

 

[duncan]

Re: NMEA - 2 GPS\'s

I would leave the 128 connected as is except the repeater, then wire the new unit at the helm into the repeater. Do not remove the wiring from the 128 to the repeater completely and if the 128 fails you can quickly rig the feed from the new unit to the autohelm if required with a couple of wire twists.

you now have the 'exposed unit' (to both knocks and weather) outwith your critical functions and everything happy............ /forums/images/graemlins/smirk.gif

 

[Oldhand]

Re: NMEA - 2 GPS\'s

You are a lucky man having an AP Mk6, its got more than one NMEA output so you can have 2 separate NMEA listner systems from the one unit.

 

[JerryHawkins]

Look at this web site...

Have a look at http://www.actisense.com/

They have what you would need to do the job "properly", albeit expensively. The NMEA switch will select the input with the "best" data and output that. All depends if you've got spare cash burning a hole in your pocket, or like most of us, you could find a long list of more mundane items that need money spending on them! In which case just make up a manual switch...

Cheers,

Jerry

 

[pvb]

Not sure that would work...

The Actisense unit apparently selects the data to output by determining "the highest priority device with valid data, based on one of three reasons - a failure to send any NMEA data, an invalid NMEA sentence checksum, or if the validity flag(s) contained within the NMEA sentence indicate that this data is not valid and should only be used with caution (for GPS and Depth data)."

If you have 2 properly functioning GPS units connected to it, I can't see how it could choose between them. Same situation as with a multiplexer, isn't it?

 

[Talulah]

Re: Not sure that would work...

I don't know the Garmins but on the Raymarine 300 you can set the GPS unit to be a repeater. I have two GPS's on the network both capable of providing the NMEA data. The Raynav 300 is set to repeater mode so there are no NMEA conflicts. If the other GPS fails I can set the Raymarine unit back to master.

 

[Mariner]

Re: Not sure that would work...

What happens with the Miniplex multiplexers is that you set one channel as the primary and another channel as secondary. Now if no data is received on the primary channel for say 10 secs the multiplexer will automatically start transmitting data from the secondary channel.
Simple really.

Martin

 

[jfm]

Re: NMEA - 2 GPS\'s

Oldhand, I dont think that's any big deal. You can wire several listeners to a single talker anytime you want. Just wire them in parallel. The Mk6 just has two NMEA output connections on that back, but they're only wired in parallel with one another, nothing special.

 

[duncan]

Re: NMEA - 2 GPS\'s

I must admit I was somewhat surprised at the statement in the initial post that you can only have 3 listeners....?

 

[RalphFerrand]

Re: NMEA - 2 GPS\'s

Thanks to all for your replies.

I talked to Garmin some time back and they told me that one must never hang more than three items on the back of a 128. There is some extremely expensive box of tricks that allows more but it is beyond my means hence the switching of the DCS and autohelm. I can get the wind instument to steer the boat by using the Raymarine 'sea talk' but sadly I can't be arsed to take the cockpit hatch garage to pieces and then go through the misery of trying to force too many wires through a hole full of Sikaflex!

 

[hightech]

Re: NMEA - 2 GPS\'s

The Actisense unit prioritises on port number, i.e. port 1 has priority over port 2, up to the 4 ports.

Have a look at www.internet-marine.com/shop they supply these units.

 

[Kantara]

Re: NMEA - 2 GPS\'s

Will that be the reason I no longer get a position on my DSC after adding an NMEA feed to my Autopilot? I now have NMEA feeds to autopilot, master Raytheon chartplotter and slave chartplotter from my Garmin 120 GPS.

 

[hightech]

Re: NMEA - 2 GPS\'s

Chris,

Basically it is down to the "driving" capability of the "talker". Most will drive three loads without problem (but not all and does depend on the listeners requirements, adding an additional will degrade the quality of the signal to the point where it becomes mush.

The only way around this is to amplify the signal early on and then transmit it to your multiple "listeners"