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I'm crew on a boat (wooden 8.5m from 1973) which has a GPS and a tiller pilot that seem to work fine separately, but don't seem to want to talk to one-another. My captain is adamant this combination "used to work, but is not working now"; I myself have never attempted to use the autopilot, nor seen it in use.

The GPS is a Garmin GPSMap 182C which talks NMEA 0183 v3.0; According to its documentation, it sends "GPRMC, GPGGA, GPGSA, GPGSV, GPGLL, GPBOD, GPRTE, and GPWPL (as well as PGRME, PGRMZ, and PSLIB)".

The pilot is a Tillerpilot TP32 which also talks NMEA 0183. According to its documentation, it accepts "APA, APB, RMA, RMB, RMC, BWR, BWC, VHW, XTE (as well as VWR and MWV)".

As you'll notice, these abbreviations are helpfully incomparable, and I haven't become wiser by studying references.

The wiring connections I have all quadruple-checked; the only thing I cannot check is the actual data content of the NMEA signal data. I have also triple-checked that the NMEA settings are sane (GPS Port 1 out: NMEA v3, 4800 baud; GPS Port 2 out: None; and yes it's port 1 that's connected through to the tiller pilot's socket). I have been unable to determine the baud rate of the Tillerpilot, and the parity settings of both devices, but I must assume they are set to a common standard.

So my questions are:

  • Are these two devices, in fact, compatible?
  • If they are, what more can I check?

If necessary please advice in comments for the following:

  • Should I post this else instead, and if so - where?? (There seems to be no good fit for questions of an nautical-electrical nature. I posted to Meta, which suggested either Outdoors or Electrical; the latter just closed my 1st try as off topic so I'm also trying here.)
  • Are there any better tags for this question here?
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    Difficult to fit this question you're right, have you tried contacting the two manufacturers? Regarding your actual SE questions - don't worry the community will always help update tags, and if the post gets closed we'll try and help you find the right location for it to go to (maybe these should be a meta question about your question not in your question) – Aravona Apr 18 at 9:35
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    @aravona wow that's the most helpful outlook I've met on any stack regarding possibly-off-topic questions. Thanks! – KlaymenDK Apr 18 at 9:46
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    Trust me, we probably get the most 'possibly' off topic questions, we try and roll with them - this completely not my area of knowledge but a quick google came up with not a lot more than you already know I'm afraid. – Aravona Apr 18 at 10:13
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    I am researching through the various standards, but a bit maxed on other stuff so it may take me a wee while. This is where I started though, so you can begin: gpsinformation.org/dale/nmea.htm – Rory Alsop Apr 18 at 13:23
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    On page 28 of the Tiller's manual is the device's interface specs: 4800 bauds, no parity, 8 bits, 1 stop bit. Now you need to make sure the Garmin's interface is set to the same. – Techie_Gus Apr 29 at 12:48
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Garmin GPSMap 182C and Simrad TP32 share the same language (NMEA 0183), but they do not have enough in common to talk about.

The NMEA sentences that the 182C can transmit are GPRMC, GPGGA, GPGSA, GPGSV, GPGLL, GPBOD, GPRTE, GPWPL, PGRME, PGRMZ, and PSLIB. The first two letters of these identifiers are the Talker ID. GP stands for GPS receiver. PG and PS belong to Garmin proprietary sentences. Let's remove the talker ID, and forget about proprietary sentences, then we are left with:

BOD, GGA, GLL, GSA, GSV, RMC, RTE, WPL

The TP32 understands

APA, APB, BWR, BWC, MWV, RMA, RMB, RMC, VHW, VWR, XTE

The only sentence they have in common is RMC (Recommended Minimum Navigation Information). Through this sentence, the TP32 can receive the time, the position, the speed (over ground), and the log (distance sailed, over ground).

The TP32 is an autopilot. It requires some input about where to go. Normally this information is provided as a bearing and/or cross track error:

  • The bearing could be provided through one of the sentences APA, APB, RMB, BWR, or BWC.
  • The cross track error could be provided through one of APA, APB, RMB, or XTE.

The 182C on the other hand side is less flexible. It cannot transmit the cross track error. It can transmit only one sentence that contains the bearing, that is BOD (Bearing - Waypoint to Waypoint), but the TP32 does not listen to BOD. These two devices are compatible on protocol level, but connecting them is - unfortunately - useless.

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    Excellent first answer - welcome to the fold! I hope to see more of these in future. – Toby Speight May 22 at 10:17
  • That is indeed a good answer, it makes me wonder - is the OP missing a piece of equipment? I mean, a "cruise control" gadget could be missing. There are compasses that output heading and desired heading as electronic signals, aren't there? – Stian Yttervik May 28 at 8:45
  • @StianYttervik Yes, good point, but normally only GPS devices and chart plotters are sources for bearing+XTE. Remote display/controls for auto pilots don´t generate waypoints or calculate bearing/XTE. No idea what happened on the subject boat. – bogl May 28 at 20:43

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