NMEA nonsencetences !

[trowell]

Good eve. Today I connected my transponder and VHF to my plotter. All went well with, I might add, some excellent telephone help for the nice chaps at Garmin. Unfortunately after a few minutes doing a celebratory jig the VHF lat 'n long turned to alot of ?????????? question marks....there was obviously a problem. I have a couple of questions. Firstly, how can i set my multimeter to check the output from the plotter? and secondly, am I correct in assuming that while sharing a port is ok the two devices doing so must be working at the same baud?

thanks in anticipation

 

[prv]

am I correct in assuming that while sharing a port is ok the two devices doing so must be working at the same baud?

You can have several listeners listening to one talker. You cannot connect two talkers together on the same wires or they will talk over each other. Listeners must always be set to the same baud rate as the talker in order to understand it.

Pete

 

[Billjratt]

The fact that the VHF knew where it was to start with would imply that there is not a baudspeed or other configuration problem there, but what is the transponder trying to do?
Could it be that the transponder came alive after the VHF and interrupted the conversation?
I would disconnect the transponder, get the VHF happy, then add the transponder to a different channel on the plotter (although you don't say what the equipment actually is, most Garmins have multiple I/O ports.

 

[nigelmercier]

You can have several listeners listening to one talker. You cannot connect two talkers together on the same wires or they will talk over each other. Listeners must always be set to the same baud rate as the talker in order to understand it.

+1

 

[concentrik]

Firstly, how can i set my multimeter to check the output from the plotter? and secondly, am I correct in assuming that while sharing a port is ok the two devices doing so must be working at the same baud?

To your first question - you can't use a multimeter in any meaningful sense on a data bus (the NMEA output from your plotter). The output is asynchronous data at a frequency well outside a multimeter's capabilities. I'm afraid you'd need a serial data analyser - but why bother? You already have the most important bit of test equipment, ie a working NMEA receiver - your VHF. Presented with coherent NMEA data it will, provided the Baud rates match, display correctly indicating the that the plotter is sending correctly. Which answers your second question as well - yes the Baud rates will need to match. Check the respective manuals and configure each item correctly. No harm will come from incorrect settings, it just won't work.

Adding your 'listeners' one at a time is a good idea.

Are you quite sure you don't have the 'DATA' and 'GROUND' connections inverted?

Is it a long cable run? Do the manuals suggest terminating the data cable? If yes to both of these, make sure you have a terminator close to the furthest 'listener'

 

[RAI]

By transponder, do you mean AIS transponder? It works on a higher baud rate than normal NMEA messages.
If that is the case, the AIS transponder output should go to the chartplotter input and the chartplotter output (GPS messages) go to the VHF input.