nmea interface problem, can you solve it ??

simonfraser

simonfraser

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Hi,

i have:

a working tacktick depth display, interfaces with GPS and TT wind, all no problem

now i have added Autohelm 1000+ to take the nmea off the TT nmea box

the Autohelm works corrrectly with the GPS and TT wind via nmea

BUT, only when the autohelm is connected to the nmea does the depth display rubish about 50% of the time

the depth suddely goes to 50 - 80 m then back to normal - 5 ish.

TT sugests that the depth signal is capable of "bouncing round the various components"

if so, would the autohelm be doing this, how do i stop it ??

many thanks for your input
 
presumably the depth display gets its info by radio but I would be very surprised if there were any controls / options on the way that signal is sent. so apart from moving the display nearer the sender or vice versa there isnt anything you can do about the wireless side of things. so about the only thing you have to play with is selection of the nmea sentences that the gps puts out and tbh I cant see that doing much.

if tactik havent got an answer I dont see you getting one here, but lets hope I'm wrong (as usual)
 
You can get interference from the cable to the Autohelm, I had a similar problem with crosstalk with the VHF that I never really solved!
 
If the cable is shielded then you could try connecting that to earth at one end - probably the TT end.

You could also try a ferrite.
 
Sounds like you have all talkers connected ..... which can interfere with each other - even when sentences are disimilar as they should be.

I would suggest that you need to check the overall nmea sentences sent .... - which means sorting out which instrument are you feeding all to effectively as your master unit.

Having read thru your post about 5 - 6x .... it's a bit strange ?? You say you took AH from TT box ... as I see it the TT box is the wind and depth ......
Why did you not take AH direct from GPS - as that is what will control the AH ... the TT stuff should only be passing info TO the GPS to possibly display on the plotter ? So I would disconnect all GPS data TO the TT stuff ... only have FROM TT to GPS..........

One point that many miss is that NMEA being simple - is that it is also easy to muck up ... just by adding in one more talker .... etc. If it's not needed to talk TO - then don't connect To side ...

Good luck ....
 
sbc,

it's more likely to be a nmea talking mess, rather than radio interference from the AH cable ??

i take the AH from the TT nmea box as it does not see a nmea signal if i connect it to the GPS, TT sugested this too

i can reduce the nmea talking by not letting the GPS see the depth from the TT nmea box, i'll try that tomorrow
 
Hi Simon,

I'm no expert but have recently installed a similar set up. Does your TT depth report to a hull transmitter. If so I dont understand why its getting into the nmea interface. The interface has two in , one out . If its any help - my set up is GPS in -Autohelm 2000 out -GPS gives COG, SOG to TT Wind( so I get almost True wing and VMG , TT wind sends apparent wind to Autopilot for wind mode. Autopilot also collects Track and waypoint info from GPS . All works well I found TT people very helpful even returning my call in the middle of cowes week .

Cheers
 
thanks, your setup is v. similar to mine, except i have TT nmea out to GPS as well as in from the GPS

this displays the depth on the GPS, and allows for a shallow alarm on the GPS

the TT depth/speed displays are too far away for the audible alarm to be of any use

i shall remove the TT nmea out to GPS to see if this solves the problem

but then, how to rig up a shallow depth alarm that i can hear ?

p.s. yes i agree TT are v helpful :-)
 
talking mess vs shielding ?

Of course interference can cause all sorts of rubbish to happen. Also having too many talkers can do same.

I would be inclined to leave all as is ... disconnect one item from nmea, still powered up though - see what happens. If still same - reconnect and then disconnect another item and so on till you get to a situation where you have no problem ... BUT when you disconnect leave plugs very close to where they connect to try and see if they pick up interference.

I am surprised that GPS doesn't work the AH though ... ok I think you want wind compensated direction to AH ... using TT gear ... but also note that AH does talk back to indicate command obeyed or not etc.

I also assume that AH works fine even when depth is going nuts ?

I have to say also that AH were notorious for Seatalk and I have heard of combined NMEA / Seatalk machines having interface problems ... but rare.

It may be a case of starting from scratch again ... and bring in each item one at a time till you get the combination / addressing right.

I am sure that others here would be very interested in you giving a post at intervals as you work through this - the good and bad till you succeed - invaluable data for others to have ...

Good luck ...
 
Simon, does you setup look like this?
nmea.jpg
 
[ QUOTE ]

IF the nmea cable to the autohelm acts as an aerial and thus interferes

how should i try and shield it ?

it has a shield already, but it's not connected, yet

? earth it, ? both ends ?

[/ QUOTE ]

The first hopeful comment. Earth the shield - it doesn no real good if it is not connected to anything much. Doesnt matter which end in theory. And if you still suspect that this cable is picking up tramp signals, then go to Maplins, but some ferrite beads (used against TV interference) and clamp them round the cable.

Having said all that, I cant see why interference should attack the depth but not the other instruments. but its worth trying.
 
Danny,

yes ! excelent diagram :-))

the AH was on the out of the GPS but no wind or course data received

the AH works fine, "just" the depth going nuts when connected

i shall disconnect the TT out to GPS in tomorrow, so the depth does not show on the GPS

was just thinking, the TT nmea box has 3 nmea interfaces, 2 out 1 in ?? not sure
anyway, it might be better to split the AH & GPS outs, 1 out to GPS and 1 out to AH
the TT website is down at the moment & manuals are on the boat :-((

regards
 
[ QUOTE ]

it might be better to split the AH & GPS outs, 1 out to GPS and 1 out to AH


[/ QUOTE ]Yes, that's definitely worth a try. If you do this make sure you ground the screens of both cables at the TT box end (and not the other end).

(Thought I'd spotted another problem with NMEA data looping round - but not so - so edited it out)
 
ok, today, after a nites sleep and a think,

i switched on the system without GPS and not connected to the nmea TT box

same problem

so it has to be something the AH nmea wire radiates

i have earthed one end, then the other, then both, doesn't make any difference error TT depth readings still occur

then went for a sail, i think it is much more stable when it's deeper, 9 m, rare round the colne river !

so is the nmea wire transmitting something similar to the depth transducer, this then bounces off the sea bed and causes the error readings ???????

how to stop this, ferrites i guess, but which ones, they sell them at maplin
 
OK so GPS disconn ... same problem ....

Have you tried disconn each item, reconn etc. ..... till you get stable situation - such that you ID the offending item ?
 
[ QUOTE ]
so is the nmea wire transmitting something similar to the depth transducer, this then bounces off the sea bed and causes the error readings ???????

[/ QUOTE ]Well depth transducers can be susceptible to noise but usually only to acoustic noise at around 150KHz - not electrical or electromagnetic noise.

How about if you have everything connected but leave the AH powered off?

Did you try with the AH connected to the second NMEA o/p?

I doubt whether ferrite beads will do the trick. They're great for suppressing high frequency noise but I wouldn't have thought an echo sounder's circuitry would have been sensitive to those sorts of frequencies (but I may be wrong about this - It's only a supposition).
 
[ QUOTE ]
depth transducers can be susceptible to noise but usually only to acoustic noise at around 150KHz

[/ QUOTE ]Whoops! I forgot. You have TackTick instruments, which are connected wirelessly, so could well be susceptible to electromagnetic noise. As regards trying the ferrite beads: if nobody has a better idea I would go for whatever fits nicely over the cable. If you have spare cable length then you could also try winding it around a large ferrite ring.
 
I wonder if it's the second NMEA input floating, try connecting the GPS to both NMEA inputs in parallel it won't do any harm, it will either work or not, and may well cure the problem.
 
ah, slightly wooly statement in my prev post :

----------------------------------------------------------

i switched on the system without GPS and not connected to the nmea TT box

same problem

----------------------------------------------

i switched it on this morning without the GPS AND without the AH nmea connection to the TT box, i.e. unplugged both, but left the wires in place

provided there is power to the AH, same problem
power to the GPS makes no difference

that is what made me wonder about the AH mnea cable emitting something

I'll try some ferrites
 
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