GPS to Autopilot: Why Don't You Understand My NEMA?

[rardiH36]

Before contacting the manufacturer, thought first to ask here for advice:

My GPS, (2002 New Horizon 150C), communicates speed over ground (SOG) and course over ground (COG) just fine to my Raymarine ST4000+ autopilot's display via the NEMA out from the GPS to the NEMA in of the Autopilot. Both units were purchased by from the same vendor on the same day by a PO in 2002.

But the Autopilot won't recognize NEMA information relating to waypoints entered into the GPS: Such as Distance to Waypoint and Cross Track Error (XTE).

The list of NEMA sentences from the GPS I can choose to send to the Autopilot includes all those (and more) listed in the Autopilot manual as being capable of decoding: Such as XTE, RMA, RMB. I've tried all of them, with no success.

Any suggestions of how to troubleshoot and fix?

Many thanks,

rardi

 

[Brice]

My guess

There may be a serial bus speed mismatch between the autopilot and the source. 9600 baud versus 19.2k. If one is expecting a certain speed and sees another, they will see garbage.

Also verify that the XMT/RCV *ARE* crossed. Meaning that the XMT of the source is connected to the RCV of the destination. Sometimes people want to connect the two liked wires together. Which is logical if your thinking about power, but not on communication devices.

 

[jviss]

Nit: it's NMEA, for National Marine Electronics Association, not NEMA, as in National Electrical Manufacturer's Association. However, most pronounce NMEA as "neema."

On you issue, maybe you have to be in AUTO mode on teh AP and have selected TRACK in order to see XTE, and bearing and range to next WP.

 

[jviss]

There may be a serial bus speed mismatch between the autopilot and the source. 9600 baud versus 19.2k. If one is expecting a certain speed and sees another, they will see garbage.

Also verify that the XMT/RCV *ARE* crossed. Meaning that the XMT of the source is connected to the RCV of the destination. Sometimes people want to connect the two liked wires together. Which is logical if your thinking about power, but not on communication devices.

It's not a comms issue since he see's COG and SOG fine.

 

[rardiH36]

NMEA More Correct Than NEMA

JViss: Thanks for the NEMA vs. NMEA correction. Transposing letters/numbers has always been a feature of my personality. In college, I could do the proper method/process to solve a difficult calculus problem. But then fail to earn an A on the test because I would do a "1+1=3" with the arithmetic. Anything important I now check more than twice.

Thanks for the tips. I will try the Auto + Track. I don't think I picked that up from the manual.


Brice: I don't recall seeing in the manuals info about baud rate and if they can be set, but I'll take another look.

again thanks.

 

[jviss]

another thing I though if is to see if the GPS is really outputting the XTE and other sentences. There are a few free programs for the PC for displaying NMEA.

 

[rardiH36]

Auto Pilot to GPS: I've Got Your NMEA Now

Dear jviss and Brice:

Thanks your tips which I did consider and I looked for info in the manuals.

But the problem turned out to be lack of user experience with GPS's. The ST4000+ instructions said if three dashes showed on the autopilot display when on the desired data page, then the GPS wasn't receiving the signal. Since the autopilot was reading SOG from the GPS, I thought that I might have a compatability issue with only some of the GPS output codes.

I didn't know that establishing a waypoint in the GPS isn't the "destination" the autopilot needed. Instead the waypoint must be tagged a "go to" destination. Once I did that, the autopilot accepted the info such as distance to waypoint.

Great to have the GPS and Auto Pilot on good speaking terms.

regards,
rardi

 

[RobertLangdirect]

Try running a NEW COMMON GROUND, even if it is temporary and just lying about, between your instruments and the battery. A ppor ground can cuse a difference in voltage potential and the instuments will read wrong or refuse to act on the signal.

If the temp ground fixes the problem, good luck fishing a permanent solution.