Anyone for an NMEA question?

[martini]

The more I look into NMEA, the more confused I get!

The three connections I want to make are:

1) compass tx > gps rx (compass heading data)

2) vhf tx > gps rx (dsc position data from other vessels)

3) gps tx > vhf rx (broadcast my dsc position data)


The units I have are:
gps - Garmin 5012
vhf - Icom M505
compass - KVH Azimuth 1000


The relevent wire colours etc I have are:

compass tx port = green (+), brown (-)

vhf tx port = white outer sheaf (-), white inner core (+)

vhf rx port = red outer sheaf (-), red inner core (+)

gps tx port = grey (A), pink (B)

gps rx port 1 = white (A), white/orange (B)

gps rx port 2 = brown (A), brown/white (B)

Does anyone fancy having a shot at explaining the required connections to me?! The bit where I get really lost is when some manufacturers use + and - while others use A and B. And I'm not sure whether + goes to - or + etc.

HELP!

[Bigmuz7]

Sharing your pain Martini .. the last time I asked the question was on this thread as follows if you havnt seen it already clicky which may offer some help from the boys re colours.. etc

[jwalker]

You'll need to read your Garmin manual carefully to see how they have implemented this. For a while now, the companies producing navigation equipment for, lets say, a 'non-commercial application' have used a variation of the RS232 system where ground is used for one side of the connection and anything above about 3.5v is recognised as a 'high'. However, the A - B system swings past 0v so the signal requires a swing from +ve to -ve. From memory, I recall it is an RS422 port and +12v to -12v but that may be brain fade on my part.

Now, I believe Garmin have introduced this into their latest equipment but also continued to implement the old system too so you'll need to check out how they have done that.

[jwalker]

Here ya go...

[martini]

That all went completely over my head but thanks! At least I got from the pic that A is + and B is -!

[jwalker]

From your description and Garmin's instructions:


For normal NMEA (RS 232)

VHF centre core white to 5012 white
VHF centre core red to 5012 grey
5012 pink n/c
5012 orange/white to ground

I guess the VHF outers will connect together and then to ground (Garmin Black)- Check your VHF fitting instructions.

The 5012 may have another channel also...I'll check.

Edit: 4 receiving ports and 2 transmitting ports. It's all in the installation instructions on pages 15 onwards. If you don't have it, go to Garmin's web site and download a copy.

[martini]

Thanks JW, between that and other bits of info I've gleaned the diagrams in the Garmin installation manual are now starting to make sense!

I am starting to wonder though if the gps will accept heading input from the compass, I can't find any mention in the book of the appropriate nmea sentences.

Does anyone else have an electronic compass hooked up to a gps?

[jwalker]

Originally Posted by martini

Does anyone else have an electronic compass hooked up to a gps?

Yup, I do. It's a fluxgate compass which is part of my auto pilot and I have also fed its output into my Raymarine radar/plotter so that MARPA will work (requires heading data.) I've not fed it into a Garmin unit but it is mentioned in the 5012 manual. I noticed it last night when looking up your other stuff.

[martini]

Thanks, I'll go back and have another look. I haven't got MARPA or autopilot, just thought it would be more accurate than gps fix at slow speed/stopped. Also less confusing if my chart is in head up orientation.

[jwalker]

Originally Posted by martini

... just thought it would be more accurate than gps fix at slow speed/stopped.

Nah, not unless it's flat calm cos the fluxgate rocks all over the place trying to keep up with the boat's movement.

Also less confusing if my chart is in head up orientation.

Do you really intend to use that mode?... Anyway, if you're pretty much electronic navigation, you'll likely have all your stuff set to true north so inputting magnetic north will befuddle yer brain at a crucial moment!

[martini]

Originally Posted by jwalker

Do you really intend to use that mode?...

No! Never have done, prob never will More a case of having 2 wires and wanting to connect them to something!

Although I do have a drift fishing spot thats often out of sight of land, when you get to the end of the drift and want to head back to the start point you have to drive off in any direction and wait for the gps to compute a heading. Would be nice to know which way before you set off.

[jwalker]

Originally Posted by martini

...when you get to the end of the drift and want to head back to the start point you have to drive off in any direction and wait for the gps to compute a heading. Would be nice to know which way before you set off.

Make sure 'tracks' are set to on and you'll know where you are and where you've been...a Garmin unit is spookily good at knowing which way you are pointing. Just follow them home.