Currently I feed my gps output to my laptop (running maptech charts) and to the VHF (DSC). I have to switch the kit on in the correct order otherwise the VHF cannot 'see' the gps. So I switch on the gps, then when it has found itself, I switch on the VHF and finally the laptop.

I am planning to upgrade from the free maptech programme to maptech navigator pro - so that I can add AIS 'A' to the system. So far as I can see the NASA AIS engine (and probably any other engine) also requires a gps input and I believe that my gps output cannot be split again as the signal amplitude is inadequate to serve 3 user systems.

I do not wish to spend any more than is necessary (I am a Scotsman after all!) as I have already spent a small fortune fitting a new diesel engine and my wife is looking a bit sideways at spending on THAT boat. I looked at gps multiplexers at the Boat Show and they struck me as being expensive especially when added to all the other costs involved.

I believe there is a possibility that the gps fed to the AIS engine is then fed to the laptop from the AIS engine to support the AIS display. Is this true? and is this gps input to the laptop also available to run the Maptech charts? If it is correct, for both questions, then I could continue with the present system with the gps output currently going to the laptop diverted to the AIS engine.

Any advice or lessons learnt from practical experience would be welcome.

The recc'd max number of "listeners" is 4 ... but often you can connect more.

My solution would be to construct a junction where the GPS supplies NMEA in ... and you via screw terminal blocks can connect in PARELLEL all listeners.

Previously - I took a standard screw terminal Telephone socket. The GPS data and power lead came in. The socket had 12V supply in.

Listeners - PC / PDA etc. all came in via telephone plugs ! The beauty of this system - allowed tel plug doublers to be connected to expand ...

Another junction was a standard small domestic distribution box ... RS232 9 pin sockets were fitted to the sides - with one side a lead coming out to connect to PC. So GPS could plug into a RS232 socket ... inside the box all other sockets / leads were parelleled of ... you could connect what you wanted as long as they "listened".

The problem comes when you try to connect more than 1 talker ... a supplier of NMEA data. They conflict. Then you need to get one of the most misunderstood pieces of kit - A Multiplexor. This item basically switches between the talkers allowing them to have short bursts of time each so no conflict of data TX. There are people who will tell you to use Multiplexor for your problem - Wrong.

If you really have a problem - then the suggestion to get hold of a cheapo Handheld GPS ... eTrex etc. is fine. Easy to set-up ... handy as back-up ....

Your problem is a common one where a little bit of care and thought will solve easily. If you need any help on this : email me at     nluther"at"solent-life.co.uk   replace the   "at"   with @ .....

Don't despair !!

http://www8.garmin.com/support/faqs/faq.jsp?faq=68

This may be of help-refers to Garmin but shows different set ups

Phil

Just a short note about the number of listeners that can be attached:  Every listener draws a small current from the talker - otherwise it wouldn't sense the electronic signal.  The trick is keeping your draw below the maximum level that the talker can provide.  Exceed the output and you get an ever increasing error rate in the signal your listeners receive.  NMEA buffers and multiplexors (for use with multiple talkers) amplify the talker signal and also remove any signal noise.  If you don't have one, cross check the positions that your systems are reporting on a regular basis.

I have attached 2 listeners (VHF and AIS) to a break in the antenna cable between my old fixed GPS and its powered antenna.  They both get the position reliably, but I get noise fed through the power connection to the AIS alarm buzzer!  Its very quiet, but high pitched - too much like tinnittis.

Keith

I have attached 2 listeners (VHF and AIS) to a break in the antenna cable between my old fixed GPS and its powered antenna.  They both get the position reliably, but I get noise fed through the power connection to the AIS alarm buzzer!  Its very quiet, but high pitched - too much like tinnittis.

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Why did you cut the antena lead and not feed NMEA from the fixed GPS ? That way you would have had probably a cleaner signal ?

It's advised to use shielded cable when making NMEA leads - but I have found that telephone multi-core cable serves well. Even used telephone socket and plugs to create pluggable system. No noise so far ...

Interested t know if original poster cured his problem or not ? and How ?

Value of forums is not only in getting suggestions, I'm guilty as others of daft questions / answers at times !, but also the solution that OP arrives at ...... even if solution bears no relation to suggested answers !!

Folks here is how it works.

The industry standard is agreed between all parties how else could we get so many manufactures to work together? NMEA is the message format not the link.

I am assuming a propriatry marine GPS and not a laptop type plug in unit.

The link output is standard is RS232 with opto isolators on the inputs and TTL logic on the output.

Which means nice big output and very little required for the input. In theory you can connect lots to the output of one GPS as long as you dont expect them to all talk back to the GPS.

Martin McArther asks for a solution I would suggest that both the DSC radio and the AIS only need to listen to the GPS outputing position therefore they can be paralled together (blue and white check polarity). Dont connect the return data (often green).

Then consider the Laptop how has this been connected? I guess probably by USB (thats also RS232) or the D type RS232.

The first option is to run up the GPS and VHF / AIS then plug in the lap top it should all be working assuming the laptop is only listening. If on the otherhand the GPS is looking for waypoints from the laptop and the laptop to give them make sure that the VHF and AIS do not see the data on the green.

Trust it all works for you Martin McA.

Martin