NMEA - talkers and listeners - can someone explain

[skyflyer]

Further to my other thread about re-cabling my instrumentation I am looking at reducing the number of cables and simplifying or consolidating but at the same time increasing functionality if possible.

At the moment I have a DSC/VHF radio that ONLY links with the (quite old-tech) Chartplotter so each supplies the other with GPS and AIS info. It is described as being NMEA in the manual, the AIS link being 38400 baud and the rest 4800. Nothing else interfaces with this "closed loop' network at present.

From what I have read about NMEA0183 you're only supposed to have one talker but as many listeners as you like, so presumably the 38400 circuit is deemed separate from the rest of it and doesn't count. It would seem that the plotter is the talker as far as this basic NMEA0183 circuit is concerned?

The plotter has no other inputs - at the moment - for things like depth and log-speed, but it does have NMEA input connections and can display this data if it is supplied with it. So if I connect (say) the wind transducer and the depth transducer to the plotter NMEA network we now have three talkers, the wind and depth transducers and the plotter - and thats not supposed to be allowed! You can see how I am getting confused?

If we forget that and instead I want to take GPS data from the plotter to feed into (for example) my radar NMEA input port, is it simply a matter of running a positive and negative from the two wires in the closed loop to the radar or does it have to be daisy chained?

In other words if you have 3 bits of kit, A, B and C, is the correct wiring this:-

A---------B--------C

or can it be


A
|
|
+--------B
|
|
C

or either


My only thought is that maybe the chart plotter is acting like a multiplexer,

Help gratefully received!

 

[prv]

From what I have read about NMEA0183 you're only supposed to have one talker but as many listeners as you like

One talker on a circuit! Not one talker on your whole boat!

Imagine the talker is communicating by morse code, by sending on-off pulses of 12v along the wires (that's actually not far off the reality). Of course if you have two people trying to do that on the same pair of wires at the same time, nobody trying to listen to the dots and dashes will understand. But if one person is morsing down one pair of wires, there's nothing to stop somebody else sending their own messages along a second pair. And in fact, they can even share a 0v/ground/return/black wire if they like, they just each need their own 12v/signal/data/red ones. Think of all the wiring as if it was going to be used for on/off morse code this way and you won't go too far wrong.

If your plotter is sending GPS positions to your VHF, and your VHF is sending AIS messages to your plotter, then that's two separate circuits, one in each direction, each with its own talker. And you might also have a third circuit, also from the radio to the plotter, to carry DSC messages at 4800bps. Each one-way link is a circuit, each has (should have) exactly one talker, and each is independent of any other circuit. One device (for example your plotter) can have connections for multiple circuits both inbound and outbound, but they're still independent of each other from an NMEA point of view.

Don't think of it as a network like a computer network. Arguably the mesh of links is a network in a sense, but really NMEA 0183 is just a collection of point-to-point links, each carrying specific (and usually limited) types of data.

Pete

 

[rogerthebodger]

Consider a talker as an output and a listener as an input and as Pete said one output on one circuit but many listeners. The maximum number of listeners on a circuit depends on the electrical strength of the talker and the total load of all the listeners on a circuit. I is normally considered the 1 talker can drive 3 listeners but sometimes you can get away with more.

You can have any number of separate circuits on your boat and some of the devices with both talker and listener can repeat the listener message out of the talker thus multiplexing the messages.

 

[FulmarJeddo]

Your plotter will have an input and an output for each NMEA port. The out of each port on your plotter which will carry multiple information eg. position, SOG, Waypoint info etc. to multiple listeners (IN) ports. You can only connect the (out) of one device to the (IN) of each NMEA port on your plotter. It depends on how many ports the plotter has, as to how many instruments you can connect. My Garmin plotter has 2 NMEA ports. I have one set as High speed (38400) NMEA 1, with the AIS engine connected and the other at low speed (4800) NMEA 2 with various listeners connected to the (out) port (Radio, Yeoman plotter, autopilot). The Radio (out) is connected to the NMEA 2 (IN) port on the plotter and is the only talker on that port.

I believe there are NMEA Multiplexors' available that would allow you to connect multiple talkers to one port, but I have no experience of them.

You didn't mention the make of your plotter and instruments. If for instance if they were all Raymarine you could network them over Seatalk, or Garmin uses CANet.

 

[skyflyer]

I believe there are NMEA Multiplexors' available that would allow you to connect multiple talkers to one port, but I have no experience of them.

You didn't mention the make of your plotter and instruments. If for instance if they were all Raymarine you could network them over Seatalk, or Garmin uses CANet.

An old Standard Horizon CP175 plotter and a new S.H. GX2100 DSC radio with AIS. The SH plotter isn't very good at specifying which wire is which - meaning rather than use standard terminology they refer to their own proprietary products they want you to buy and connect to it.
So the ports (wires) available on the plotter are listed as:
+12v
0v
VHF NMEA output
VHF NMEA input
Fishfinder Input
Fishfinder OutputFishfinder
GPS antenna Output
GPS antenna input

from which you will gather it is not an internal GPS aerial and the GPS aerial has its own 12v/0v supply too

SH supplied me with a diagram that showed the AIS output from the radio wired to the Grey terminal of the plotter (Fishfinder input) which implies it is (or can be set to) 38400baud

But all in all the plotter appears to have no more than 3 separate circuits.

I think it may well work as a multiplexer.

The question is do I risk any damage by experimenting or will it just not work if I connect it wrong (obviously always keeping +ve to +ve and ground to ground)

 

[prv]

The question is do I risk any damage by experimenting

No - if you connect it wrong it just won't work. It shouldn't come to any harm.

Pete

 

[Deleted member 36384]

One talker on a circuit! ..... Imagine the talker is communicating by morse code, ......

Thank you, such a clear explanation, so simple really. :encouragement: