Do I need an NMEA Multiplexer?

muckypup

muckypup

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If I have a GPS with a single NMEA0183 output, and I want to pipe this into a DSC VHF set, a Navtex and an autopilot... do I need a multiplexer? How about if I wanted to connect this output to an RS232 PC port as well?

Steve
 
Hi, I'm just in the process of doing this myself and I understand you can connect up to three without a multiplexor based upon the research off this forum and below which is a summary .

My GPS II+ manual says the NMEA output will drive three devices.

The link below shows the correct wiring of a 9 Pin RS232 serial connection (85p from Maplin and 1.25 for the case covering)

http://www.fugawi.com/docs/hardwiring.html


The following website was helpful on understanding NMEA

http://vancouver-webpages.com/peter/nmeafaq.txt

And if you page down half a page on the following link you will see the wiring for a 1:3 nmea connection. As I need 1:4 (GPS Repeater, VHF, Computer and Tiller Pilot) I plan to use a microswitch (£1.8 from Maplin again !) to switch bwteen the VFH DSC and Computer as the chart plotter will not always be on but the VHF will be.

http://carlten.tripod.com/Navigation/navigation.html

Hopes this helps. Having prepared all the cable connections will be wiring most of it up at the weekend so will let you know if there are any problems.

Cheers
 
No not unless you intend to connect more than 4 items.

Secondly one of the items can always be fed via passthrough of the PC .... but make sure that one is only needed when you are using PC ....

Many people have successfully split a signal into multiple items.

I for one have used multi-core telephone cable, Maplin supplied RS232 sockets / plugs, electrical junction box, telephone wall socket etc.

one example here is a junction box with 3 RS232 connections. Data in from GPS, fly-lead out to PC, socket for PDA / other item requiring data.......>

DSCF0263.jpg


On my older Magellan ..... the data box was also the 12v power connection - so the cable was connected to a standard BT telephone socket box. 12V came into the box to respective connections and then PC / other items connected to the box via standard telephone connector and cable. It all worked and cables were over 2m long allowing siting of each item where you wanted.

To be honest and no disrespect to many - there is a lot of hocus-pocus spoken about connecting these ... where in fact as long as you don't have a high RF interference factor, use reasonable quality cable etc. - you can do it easily.

Good luck and don't be afraid to experiment .... remember that the data connections can be swapped / tried without damage - its part of the NMEA spec .....(just don't touch power to any !!!)
 
As I understand it the need for a multiplexor only arises when you have more than one OUTPUT requirement. i.e. 2 ( or more ) devices needing to write to the NMEA bus simultaneously.

NMEA outputs a signal as a "sentence" of bytes. If you have more than one output device then there must be a way of determining that any device's output sentence is not corrupted by another device "talking over" the first device's output. The multiplexor ensures that only one device at a time writes to the NMEA bus and completes its sentence before the bus is available to any other device.

The number of devices that can be driven by a single output is probably determined by the voltage drop across each device's input ( including wire resistance etc ).
 
I think that technically speaking a multiplexer is the "opposite" of what you are looking for: a multiplexer is used to combine multiple NMEA inputs into one output.
What you want is a 'driver', i.e. something that will 'source' more current to drive a number of 'sinks' (devices).

As has already been stated most NMEA ouputs (drivers) will cope with three or four devices (sinks); also as mentioned above you could use an NMEA output from one of the other devices (if they have an ouput) to drive a fourth/fifth/sixth device.

If you really need a 'driver', have a look at an RS232 driver which should also work for NMEA ......... they are likely to be much cheaper than anything with "marine" in the name (e.g. NMEA)!!! NMEA is only a serial protocol that is very similar at physical level to the serial RS232 protocol (there is a difference in levels though).

Good luck,
Alan.
 
Thats easy then. Next question... short of just twisting the wires together what do people use to connect these together?

The ideal way would be to find some small 3 pin connector plugs & sockets and wire up a small connector box. Someone above did mention using RJ45 connectors but I hate making those up (and I don't have the crimp tool).

Steve
 
YEp chocolate box (those white plastic things) crimp connectors or a junction box are the easiest ways. I wouldnt just twist together the wires and tape them, because you've only got to go over a wave and the whole lot will pack up!
 
It really depends on what RS 232 driver chip your GPS is using.
with a MAX232 then you'll probably be OK from four to six units.
For instance my Garmin 235 Plotter sounder will happily do six listeners without any problem...
viz..Two DSC radios, PC Plotter, AIS unit, Radar, and NASA GPS repeater.
No problems with this so far, but I have used proper RS232 foil shielded network
cable to do the job.
This is the key really. Ive got some spare if you want it.
The so-called multiplexers are very over-priced. If you're handy with a soldering iron you can build one yourself..ive got circuit details somewhere.
Bits from Maplin cost about eight quid.

Steve.
 
Joining ....

Telephone connectors
Screw terminals
Twist and solder
Din plugs
Small model radio plugs

Honest the connection is not as critical as some make out .... I use often whatever is at hand in my goodie box ! As long as a clean and good contact is made - thats it.

Note - when wiring up .... you will no doubt get to the headscratching part where one item doesn't read .... normally down to getting your pins reversed .... and no matter how hard you think about it - you are SURE they are not !!
My way is to get one working and have bare pins available to touch with next wires. Touch and see if the next reads and so on ... as already said the protocol standard calls for safe wrong wiring /connection to not damage item.
 
I have tried to connect a Navman 5600 to both a Silva Repeater and also to a Furuno 1621 Radar. I can get it to speak to both, but not at the same time. In other words, it will not both at the same time but it will run each perfectly when only one is connected. Any advice to enable me to run both.
 
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