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Boating:

Can We Talk?

Want to hook up your GPS to your VHF radio or Radar, but not sure how? Read on...

Background   Basic (1-to-1) Hookups   Hooking Up 1-to-Many   It Still Doesn't Work! 
OK, so the wiring's in place, and everything looks good. You power everything up, and...nothing appears on the listener's display, or it doesn't work as expected. What now?

  • Of course, the first thing you should do is go back and verify that the wires are, in fact, really connected correctly. Projects like this tend to get done at the last minute, at night, just before a trip.

  • Assuming that everything really is correctly wired, make sure that the talker and listener are both talking the same version of NMEA 183 . There are at least two active versions of the protocol, v1.5 and 2.0. Many NMEA 2.0 devices can be set to talk 1.5 if needed.

  • Check that the two devices are set to the same connection speed . NMEA is a serial communications protocol, and both devices should be set to: 4800 baud, 1 stop bit, 8 data bits, no parity. If you connected your GPS to a PC to download charts and such, you may have set the GPS port to a higher rate to talk to the PC.

  • Check to make sure that your talker is providing all of the data that your listener needs. This information can appear in various places, usually in the Technical Specifications section of the owner's manuals, and may be refered to as "output sentences" or "input sentences". If your talker can not provide the right sentences to the listener, you are out of luck. You checked that when buying the equipment, didn't you? I didn't...and here is an example of the problems this can cause:

    • I purchased a sonar which can display navigation data and other data. However, it requires the following input sentences: BWC, GGA, GLL, HDG, HDT, MDA, MTW, MWV, RMA, RMB, RMC, VHW, VTG, XTE.

    • My GPS provides the following output sentences: GSA, GSV, GLL, RMB, RMC, APB. As you might expect, portions of the sonar's nav display did not work, so I gave up the idea of using it as an alternate navigation display.
  • The last thing to do is check to see if you have inadvertently set up the talker or listener so that they can not talk . A good example occured when I added the autopilot to Toy Boat 2. After hooking everything up, I put the GPS in simulator mode, picked a start location, picked a waypoint, set the simulator speed, then set the autopilot to "navigate" mode.

    I expected the AP try to turn the motor to navigate towards the waypoint. Instead, it did nothing. After much hair pulling, I got a suggestion on the AllCoast Private Boaters Forum from Kurt Stutzman (KMAN) to hook the GPS to a PC and look at the output. My GPS had a PC connection cable, so I used Hyperterminal to view the output. Imagine my surprize to learn that the GPS, in simulator mode, sets a flag marking all sentences as invalid. As a result, the AP was ignoring all input from the the GPS. Once I took the GPS out of simulator mode, the GPS-AP connection worked fine

    When you think about it, this actually makes sense. The last thing you would want to do is to forget that you had the GPS in simulator mode, then go boating and tell the AP to go to a real waypoint. To prevent this problem from happening, the GPS was built so that it could not output valid autopilot data in simulator mode.

So there you have it - NMEA connections in a nutshell. I hope this helps!

 

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