Intermittent GPS data ...

[ParaHandy]

Last year I connected a set of 5 ST60 units (wind, depth, speed, graphic and multi) to an Autohelm 4000 through the raymarine bus. A 182 gps was connected to the ST60 multi and a JRC1500 radar connected to the nmea out of the ST60 multi. A pc based chart plotter is connected to the gps 182 as well.

All works fine except that:
a) the st60's from time to time bleep that they've lost the position input (from gps) for a few seconds but the gps does not show that it has lost satellite data and neither does the pc plotter and
b) very occasionally (but often at exactly the wrong time) the ST60 multi will not transmit heading information (from the autohelm) to the jrc radar. This is not coincident with problem a). When this happens, the jrc defaults to using the COG (which is also transmitted by the ST60 multi) info which could be disastrous if you weren't aware of what's happerning. The ST60 multi also displays the incorrect battery voltage and is going back anyway .. and the ST60 graphic nmea out never worked but wasn't needed ...

Problem a) is not that serious unless it starts to get worse which it may well do. I'm tempted to think that the raymarine bus isn't up to the job or that the aerial is mounted too close to the outboard on the pushpit - seems to be less prone on one tack than the other but that might be bolx ...

Problem b) is much more serious. The multi display was chosen entirely because it took the autohelm heading data and transmitted the nmea HDG sentences to the radar which uses this info for the EBL.

Anyone come across any of this and know a cure?

 

[boatmike]

I have to start by saying I am not an expert but I have had similar problems. What I was told is that there is a limit to the number of instruments you can "slave "off one GPS. Perhaps if you tried disconnecting some of the less essential instruments it might make the others more reliable?
What I have done is leave the existing GPS feeding the chartplotter and other instruments and have a second GPS set connected to the PC I use for planning which seemed to be causing the problem. I would think the location of your antenna is a red herring or the GPS would show a lost signal wouldn't it?

 

[ParaHandy]

I'm almost 100% certain since the pc plotter is relatively new that this happens (problem a)) whether the gps is connected to the pc chartplotter or not which leaves the gps connected only to the st60 multi ...

 

[aluijten]

You might consider to use shielded cables, or if you do this already, check if shielding is OK.
Also take care with moblie phones and handheld VHF sets as they can kill off the NMEA signal. As most of the instruments are made of plastic, they can be very sensitive to RF signals messing up the internal circuitry.

Arno

 

[boatmike]

One other thought then. As the system includes a "graphic" read out, does this mean you have that instrument configured as a GPS read out? If so I would try connecting the GPS directly to that and let the Multi get it's info from the bus that it is connected to. I seem to remember with mine that some units should be configured as "masters" and some as "slaves" Perhaps your slave is driving the master (sounds like er indors dunnit?) I have also got into trouble connecting two read outs to one info source. perhaps this is it? In short connect the GPS input to an instrument that is GPS only. connect the output from that instrument to the multi. (the bus should do that) and connect the chartplotter to the GPS readout either by bus or by direct NMEA in NMEA out and see if that works. I would not connect the PC until the rest works as this is where I had problems with it cocking everything else up. ......I think......... /forums/images/graemlins/crazy.gif

 

[Oldhand]

I have gone through the on-line ST60 manual and nothing in there seems relevant to your problem, except perhaps check that NMEA is selected "On" in the User Setup. If I remember correctly the NMEA connections to the Multi are made with spade terminals which can be unrelaible if either the incorrect size is used, they are not a tight fit or if there is any corrosion. I can only suggest you carefully check these terminals and the quality of the crimps onto the cables.

Data "chasing its own tail" can be a problem but from the description of your installation you don't seem to have any data loops where an output is fed back as an input. Path and type of cable used for NMEA could be a factor, a twisted pair was originally considered good enough but there were less EMI generators in those days. Your PC based chart plotter NMEA connection could be transmitting conducted EMI on the NMEA bus which could be affecting the Multi, although I don't have such a problem on our boat. Have you got ferrites on your cables close to the equipment to supress EMI, including the power cable into the PC? Also if your NMEA cable to the Multi from the GPS is long, could the wire guage be too thin?

Hope this gives you something to think about.

 

[Phoenix of Hamble]

Para, just a thought..... if any of the devices use NMEA differential vs single ended connectivity, then another factor could be the consistency of the power supply, eg a floating or varying voltage between the two NMEA wires.... does the outage coincide with any other electrical activity such as a fridge switching on or off etc? and do you have a big enough cable supplying the power to the GPS to hadle the load?.... worth a thought at least...

 

[ParaHandy]

many thanks for your help.

boatmike: I can't without a huge upheaval reroute the gps into the graphic nmea in. the st60 graphic is in the cockpit with wind, depth and speed. the multi is over the chart table. the st60 graphic is indeed used as a gps repeater amongst other functions - it's quite useful - and all four st60's were bench tested (not the multi - it came later), with the 182 gps connected, for many hours before fitting.

oldhand: if there's an obvious cause, that's it. but each of these crimp connectors had soldered wire ends so should be ok. i'm going to check though ... the cable runs from the gps to the multi are very short as is the run to the radar.

nas: i tend not to sail with fridge etc running. there ought not to be any power instability.

aluitjen: on balance, i wouldn't suspect rf although not discounting it ...

i do wonder if the st60 multi is less tolerant (ie reacts immediately) to a dropped group of nmea data (the garmin 182 transmits every 10 seconds, I think) than, say, jrc or chartplotter which just uses previous data for a short time and not longer than its own internal software triggers a fault condition before the nmea data restarts although, in the case of the jrc, it defaults to the last cog data. in other words, the more likely might be a bad connection to the multi from the gps ...

 

[Oldhand]

I am interested to see your Garmin only transmits once every 10 seconds. Normal NMEA data transmission rates are once per second. I wonder if the slow data rate is making the Multi think it has lost NMEA input on some intermittent basis which may be determined by what is going on on Seatalk, i.e. a busy Seatalk message period may delay NMEA update beyond your 10 second input. You could ask Raymarine what they expect the NMEA input "time-out" to be.