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  NMEA 0183 Conecting Two Devices to One Port

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Author Topic:   NMEA 0183 Conecting Two Devices to One Port
6992WHALER posted 10-29-2011 11:14 AM ET (US)   Profile for 6992WHALER   Send Email to 6992WHALER  
I have been rearranging the locations of my electrics and finishing the NMEA 0183 communications between them all. When I am done I will have no ports left on my GPSs for any future connections. I was wondering if I could hook two devices that only receive information from the GPS to the same data out port on the GPS?
jimh posted 10-29-2011 11:29 AM ET (US)     Profile for jimh  Send Email to jimh     
In NEMA-0183 serial data connections it is generally assumed that two LISTENER devices can be connected to a single TALKER output. I believe that up to three LISTENERs could be connected to a single TALKER without anticipation of problems.

The details of the NMEA-0183 input (or LISTENER) and output (or TALKER) ports vary with particular devices. What I have often seen is the output signal is generated as a single-ended signal, that is, there is a NMEA output wire and its signal is referenced to the device's ground or power negative connection.

In my experience it is also common to see that the inputs are configured as differential inputs, that is, the input has two wires, one for the non-inverting input and a second for the inverting or reference input. These are often marked with a "+" and "-" (plus symbol and minus symbol) to identify them. Note that these are not polarity markings like you would find on a battery. The "+" is the non-inverting or normal logic input, and the "-" is the inverting or negative-logic input.

The normal wiring arrangement is as follows:

--the single output wire is connected to the non-inverting input wires;

--the inverting input wires are connected to the output device chassis or ground or power-negative lead.

If one of the LISTENER devices has a single-ended input, you would just connect that device's single wire to the single wire output lead.

If the TALKER device is a differential output, that is, if it has two wires for the output, you can connect the inverted output (the "-" output) to the inverting inputs (the "-" inputs). IMPORTANT: do not connect a two-wire output such that the inverted output ("-" output) is tied directly to ground, chassis, or battery negative. If there are no inverting inputs, just leave the inverted output wire floating, that is, insulated and not connected. As I mentioned, in most cases the device output will be a single-ended signal, that is, be on only one wire, and this problem will not arise.

jimh posted 10-29-2011 11:42 AM ET (US)     Profile for jimh  Send Email to jimh     
For an excellent and detailed explanation of NMEA-0183 serial data connections, I recommend the white paper on that topic from ActiSense. You can obtain it online from

http://www.actisense.com/Downloads/TechTalk/NMEA%200183/ The%20NMEA%200183%20Information%20Sheet.pdf

They show in more detail the connection arrangements that are recommended. I know the paper is a bit more than most boaters want to study for these interface connections, but it covers this complicated subject very well. In particular, see the section under the subheading

Connecting NMEA talkers to NMEA Listeners

It shows the four possible arrangements that can occur.

6992WHALER posted 10-29-2011 12:09 PM ET (US)     Profile for 6992WHALER  Send Email to 6992WHALER     
Thanks Jim,
It sure would be nice if my GPS told me I could hook up more then one device to a single port.
The article will be on my reading list.
jimh posted 10-29-2011 12:56 PM ET (US)     Profile for jimh  Send Email to jimh     
To be clear, you CANNOT connect two TALKERS to a single LISTENER. To accomplish that you need an accessory multiplexer.
6992WHALER posted 10-29-2011 06:31 PM ET (US)     Profile for 6992WHALER  Send Email to 6992WHALER     
I'm glad you clarified. I assumed if it was possible it could only be from one talker to two listeners.
6992WHALER posted 12-12-2011 07:27 PM ET (US)     Profile for 6992WHALER  Send Email to 6992WHALER     
I was looking at my Garmin 3206 installation manual and stumbled over a statement that says you can connect three listeners to one talker.

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