Re: [Amel Yacht Owners] Heading Sensor vs. Solid State Compass???

Brent Cameron

Your absolutely correct Ryan. Total brain fart on my part. I actually knew that. I plead winter separation from my boat! :-). 

On Mar 15, 2018, at 10:29 AM, Ryan Meador ryan.d.meador@... [amelyachtowners] <amelyachtowners@...> wrote:


Hi Brent,
Thank you for the deep dive on the history of these communication protocols.  There's a lot of good information there, though there are a couple of errors I feel I should correct.  NMEA2000 is not Ethernet-based, it is CAN-based.  The consequence of this is NMEA2000 has its own standardized round connectors, not the familiar RJ45 connectors most people know from their home internet router.  To my knowledge, there is no industry standard Ethernet solution for marine instruments, though there is one under development called OneNet.  Raymarine's proprietary SeaTalkNG is their NMEA2000 extension, and SeaTalkHS/RayNet is their Ethernet technology.

My reading of the Airmar 220WX specification sheet says it supports both NMEA0183 and NMEA2000, so Duane should have plenty of options for interfacing.  I'd recommend NMEA2000 because it's the more modern standard and can have multiple devices on the same bus (cable).  That same document lists a bunch of heading information that comes from the unit, so I think it would suffice for the radar on a technical level (this could be confirmed by comparing the needed data messages in the radar manual to the output messages of the 220WX), though practically might have error introduced by being at the top of the mast.

SM 233 Iteration
Boston, MA, USA

On Thu, Mar 15, 2018 at 9:49 AM, Brent Cameron brentcameron61@... [amelyachtowners] <amelyachtowners@...> wrote:

Not (yet) an Amel owner but I have a fair amount of experience wiring up and debugging electrical systems (including NMEA marine systems) so thought I might be able to help.  I’ve learnt untold new things on this excellent forum about the Amel I hope to buy in the next 12 months so want to give back.

Excluding sending charts/radar themselves, there are 4 usual interfaces of various marine systems that are generally put under 2 main categories in two flavours, NMEA and SeaTalk.  NMEA is an industry standard while SeaTalk is RayMarine’s proprietary implementation of NMEA (implements NMEA but adds proprietary(and encrypted)  extensions of questionable value - and I say this as an owner of almost solely RayMarine electronics on my sailboat).   The main categories are the older serial interface (NMEA 0183) and the newer Ethernet based NMEA 2000 interface.  NMEA 0183 (and legacy SeaTalk) is based on a two wire RS-422 hardware standard.  It’s slow (but high speed isn’t necessary anyway) but has disparate connectors for every device so usually results in splices and lose wires that can be problematic if not implemented properly.   NMEA is an open standard (so is supported by everybody) while SeaTalk is generally only supported by RayMarine (and a few vendors who make interface boxes).  Both SeaTalk and NMEA0183 can co-exist over the same wires but they send information under different formats and you generally need an interface box to be able to read and write to both.  Some GPS/ChartPlotter’s (like RayMarine’s) output NMEA0183 in addition to SeaTalk so it’s possible to use other devices like computers to read information generated by SeaTalk devices like wind instruments and autopilots.   Generally SeaTalk allows extra information outside the NMEA standard information to be sent to Raymarine head units but third party devices like computers generally don’t need access to that information anyway and the basic information available over the NMEA 0183 is adequate.  NMEA0183 is a bit more finicky and troubleshooting can be more troublesome due to the wiring challenges.

NMEA 2000 (and SeaTalk NG (Next Generation) send information over a different high speed Ethernet based network (like the hard wired internet connection on your computer).  It uses the same RJ45 six pin connector and the information traverses the network using the IP (Internet Protocol) standard.   Again the main difference between SeaTalkNG and NMEA 2000 is a few proprietary (and encrypted) extensions by SeaTalk but they can co-exist on the same network and connections.  They usually just plug right into the connectors on the back of the devices so can be easier for non-techies to implement.

From a quick review of the Airmar 220WX website, it appears that this device transmits information on the old NMEA0183 serial network.  Since it is only sending wind/gps information, this is fine as it can still update every second without challenges.  You don’t say what Chartplotter you got but most support NMEA0183 while newer ones also support NMEA2000.  

I note that this device does send heading and GPS information in addition to the wind information so your chart plotter (if it reads NMEA0183), should be able to get the heading information from that device without any challenges as the heading information is sent using a standard string whether it comes from a solid state compass or a traditional one.  A bigger challenge might be how accurate that information can be at the top of a 65’ mast rocketing back and forth over a wide arc in any sort of seas...   The site says that the unit is stabilized but I’d be a bit sceptical about that as most of their pictures have it mounted fairly low (except for the one sailboat where it is also mounted beside a traditional wind instrument).   That said, it might be worth picking one up and trying it out as it should be possible to correct for much of this using electronics - that said the more corrections you have the more chances for error.  

Brent Cameron
Future Amel owner.

On Mar 15, 2018, 8:02 AM -0400, sailor63109@... [amelyachtowners] <>, wrote:
Airmar 220WX

Join to automatically receive all group messages.