NMEA in - Can 2 devices supply one input

[BelleSerene]

No need to worry, If your DSC radio is not getting any NMEA input it will soon tell you.

Dont know why you think it would do that? Mine currently has none (don’t ask) and doesn’t complain in any way. Just shows ‘———‘ for lat, long and time.

 

[Daverw]

My last three, 2 Icom and SH all had alarms when GPS was lost after a short delay and this would have to be acknowledged

 

[lpdsn]

Dont know why you think it would do that? Mine currently has none (don’t ask) and doesn’t complain in any way. Just shows ‘———‘ for lat, long and time.

My SH starts off like that, but then it gets very loud and insistent.

 

[ShinyShoe]

Dont know why you think it would do that? Mine currently has none (don’t ask) and doesn’t complain in any way. Just shows ‘———‘ for lat, long and time.

Is that not because you don't have a position ever. So doesn't actually expect one. Once it has a position and it stops being updated most will alarm.

If you then hit the red button it WILL still send. It will include the last pos and time u to 4hrs old. It will include your MMSI

Returning to the OP, I'm inclined to ask what the risk of failure is... and what the risk of a switch failure is... are you adding more points of failure rather than redundancy?

 

[benchmark]

Just wire your gps signal leads from the GPS to the most reliable of your plotters as a Source . Keep it simple and avoid potential problematic bottlenecks.
How often has your plotter lost it’s gps signal?. Maybe Occasionally during startup but if yours has a habit of stopping while you sail,then use your time instead to look into that problem , maybe relocate the antenna away from potential interference or just get yourself another reliable model.
I understand your fears /arguments to wire 2 plotters up as a backup but that is just overkill , there are a million other more likely scenarios that can go wrong at sea.

 

[benchmark]

Dont know why you think it would do that? Mine currently has none (don’t ask) and doesn’t complain in any way. Just shows ‘———‘ for lat, long and time.

You may have inadvertently deactivated the alarm in the settings menu. Every DSC radio I have come across to date ALL have alarms to let you know that the DSC can not be used. I think it’s a legal requirement .

 

[PaulRainbow]

No, they each have their own built-in GPS receiver.

Pete

You might need to shout Pete, that information seems to have gone unnoticed.

 

[Yeoman_24]

Thanks to every one for thoughts, opinions and suggestions. Plenty for me to think about

 

[PaulRainbow]

Thanks to every one for thoughts, opinions and suggestions. Plenty for me to think about[/QUOTE]

If you discount talk of NMEA 0183 networks and external "antennas" you have little to think about.

You cannot connect them both together, whether or not they are both switched on at the same time.

You have no NMEA networks, so no way to follow those suggestions.

You have no external" antennas" (unless they were prehistoric they would be receivers anyway). Both of you plotters have internal GPS receivers. If they were both external it wouldn't really help, as they would be connected by plugs and it would be technically more difficult to connect both plotters and the VHF than it currently is.

So, how to fix it. A single pole changeover switch is simple and will work, because you have Garmin plotters.

Another option has been mentioned, fitting a pair of diodes. This doesn't address the data collision issues if you have both plotters turned on. It addresses a problem that doesn't exist. No damage would occur if they were both connected, it just won't work.

A double pole switch will also work, but you don't need the second pole for the data lines, as Garmin rely on the -ve. What would be better use of a double pole switch would be to have the second pole switch the plotters -ve. This means that you cannot accidentally have the wrong combination of plotter/switch selection.

 

[bedouin]

If we can allow a bit of thread drift - it would of course be an ideal application for a PIC. It could listen to both transmitters and only fail over to the 2nd if there is no signal on the first. You might even be able to do it with an op amp and a handful of components.

But if it is part of a safety-critical system then I would not seriously suggest that - at least not without having a manual switch as well.

 

[BelleSerene]

I have a vhf radio (Silva) with a single NMEA 0183 'in lead'. I am going to link this to an NMEA out on my main Garmin GPSmap820 plotter to allow positional DSC on the radio. All straight forward so far!

I also have a back up Garmin GPSmap 551 which I was thinking about also wiring an 'out' into the same VHF 'in' connection, the idea being if the main plotter packs up, I have a backup linked to VHF.

The questions I have are;
Will this be ok if I only have one plotter on at a time?
What will happen if I have both plotters on at the same time?
Finally, what should I attach the NMEA 'screen' wire too?

Many thanks,

What you call your ‘main plotter’ is also the boat’s GPS source. (I’m sorry; I hadn’t appreciated this at first; it has an internal GPS head and your instruments are apparently not being fed by another one.). So this ought to be always turned on while the Nav instruments are on, so that your radio, if nothing else, gets lat/long and time info.

You also have a ‘backup plotter’. The ostensible problem is that the main plotter may fail. That’s really not a serious risk, and is far less of a risk to seamanship than the problems introduced by manual switches between data outputs of two plotters. Just rely on the GPS of the main plotter, and its data feed to the radio.

If the second plotter is helpful to you, by giving you chart table-based plotting, then great. You still don’t need to feed its data output to your radio, as the main one already does that. If it’s not helpful in its own right, clear the clutter.

 

[PaulRainbow]

What you call your ‘main plotter’ is also the boat’s GPS source. (I’m sorry; I hadn’t appreciated this at first; it has an internal GPS head and your instruments are apparently not being fed by another one.). So this ought to be always turned on while the Nav instruments are on, so that your radio, if nothing else, gets lat/long and time info.

You also have a ‘backup plotter’. The ostensible problem is that the main plotter may fail. That’s really not a serious risk, and is far less of a risk to seamanship than the problems introduced by manual switches between data outputs of two plotters. Just rely on the GPS of the main plotter, and its data feed to the radio.

If the second plotter is helpful to you, by giving you chart table-based plotting, then great. You still don’t need to feed its data output to your radio, as the main one already does that. If it’s not helpful in its own right, clear the clutter.

What's wrong with having a backup on a changeover switch ?

 

[prv]

You might need to shout Pete, that information seems to have gone unnoticed.

I am not responsible for the opinions of numpties

Pete

 

[fisherman]

I made a distribution hub. Simple household blank front box with 4 gold audio connectors, plug in ready made audio leads. One GPS supplies it, and radar, maxsea, DSC plug in the front. The other gps can supply it by plugging in the front, or any item can connect to any other in emergency. It is very effective, I was told on here it would corrode but six years on no trouble.

 

[GHA]

If we can allow a bit of thread drift - it would of course be an ideal application for a PIC. It could listen to both transmitters and only fail over to the 2nd if there is no signal on the first. You might even be able to do it with an op amp and a handful of components.

But if it is part of a safety-critical system then I would not seriously suggest that - at least not without having a manual switch as well.

Or an esp8266 and send the multiplexed feeds over wifi as well as serial ...

 

[ShinyShoe]

What's wrong with having a backup on a changeover switch ?

Without knowing the failure rate of the GPS(es) and the switch - who knows? It may increase the risk of a complete failure...

If I understand correctly lets say the GPS has a failure rate of 1 in 1000 (i.e. if you sail for 1000 day sails you'd expect it to fail once during that). If you just wire direct to the radio that remains your failure rate.

If you add a single switch (lets ignore the other GPS for now) that also has a failure rate of 1 in 1000, you now have 2 in 1000 chances of failure, the switch or the GPS.

If you add the second GPS and it has the same failure rate then the chance of BOTH GPSes failing at the same time* becomes 1 in 1,000,000, but the chance of switch failure remains 1 in 1000 (100 in 1,000,000). So the overall chance of failure is 101 in 1,000,000 (i.e. 1.01 in 1000) so actually marginally higher than the GPS hard wired.

Now I suspect the switch failure rate may be more like 1 in 10,000 and the GPS failure rate more like 1 in 100. In which case dual GPS failure becomes 1 in 10,000, and the risk of failure becomes 2 in 10,000 (1 in 5,000) vs 1 in 100 so it reduces the risk of failure 50 times.

Without knowing the failure rates - you'll never know


*assumes the cause of failure is chance and not something that actually triggers failure like a power surge, satellite crashing into the boat etc!

 

[ghostlymoron]

I'm not quite sure what you're trying to achieve. AFAIK nmea allows up to 4 listeners but only one talker. Obvious really as two different sets of data to a receiver would cause confusion and possibly damage. Why not connect what you consider the most reliable to your plotter and have the other on rigged ready to go. If you get a failure, just change the plugs over and Bob's your uncle. The chances of having to do it are slim.

 

[fisherman]

80 days this year GPS failures four. Cause not known when one went wrong, 3, sat failure on both 1. The three failures involved one GPS being 300 yds away from the other, consistently for 30 mins. Mystery.

 

[Yeoman_24]

/QUOTE]

So, how to fix it. A single pole changeover switch is simple and will work, because you have Garmin plotters [/QUOTE]

That sounds like just the solution I need and is what I will do. 3 quid for a switch. Excellent.