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Old 13-07-2015, 12:50   #31
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Re: Starting from scratch

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Originally Posted by mstrebe View Post
The core problem with N2K is the limited bandwidth (it's only 265Kbps) being exacerbated by "blind senders" which stomp on the bus instead of waiting for silence, thereby talking over the reports of other units.

Ultimately yes, it's bad software design by vendors and bugs causing transmission storms.

The "right" way to solve the problem is with N2K bridges to isolate bad talkers, but just figuring out who those bad talkers are is problematic. Ultimately the real solution will be the same as it was for Ethernet: True fire-walling switches (multi-port bridges) that can isolate traffic and eliminate overzealous reports from chatty talkers, and prioritize important traffic such as information needed for auto-piloting, depth sounders, etc. The switch would simply replace the major backbone.

The CAN bus used by NMEA2000 is the same as J1939 for Semi-trucks and their trailers, but running at half the speed. The PHY layer chipset is $2 in quantity. No reason why someone couldn't build a device that solved all these problems using off-the-shelf components. A simple ARM-based logging device with an "access list" that prevented unnecessary blind senders from storming the bus would solve most if not all problems.
I'm not sure what you mean by "blind senders". I'm pretty sure the media access is arbitrated and controlled by the CAN bus spec and chips. N2K really just defines a set of messages that are specific to marine applications.

All the problems I have seen are devices sending the wrong messages at the wrong time. I have never seen any messages getting stomped on or corrupted or anything that suggests that the underlying CAN bus technology is problematic. Just the opposite. Can bus has been used with great success in other applications.
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Old 13-07-2015, 15:51   #32
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Re: Starting from scratch

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Hi Matt. How does your 12v pc, plus monitor compare in real life power consumption to a small laptop? I can definitely see the benfits of a small fixed unit in terms of, dampness and space. We used one on the 60 footer I sailed in Antarctica, with a cheap silicon keyboard. Worked great and freed up lots of space on the chart table. Wet hands were not an issue.

Sent from my HTC_0PCV2 using Cruisers Sailing Forum mobile app
Small laptops vary quite a bit, so it's hard to say without a specific example. Our PC unit is 10w working (with SSDs). The monitor is more - 20w for a 15" touch screen, for example. In my own install, I use this combination, so 30w total. That is approx. 1/2 of what my old I3 Acer Laptop with same size screen used. Also remember that the laptop needs power adaptation to get to the correct voltage, which causes some additional loss. The PC is 12v, so it does not. It has a 12v power take-off for the monitor as well, and the touch monitor is IP56 water resistant. I've been very pleased with the change from a laptop...

Cheers
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Old 13-07-2015, 19:13   #33
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Re: Starting from scratch

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I'm not sure what you mean by "blind senders". I'm pretty sure the media access is arbitrated and controlled by the CAN bus spec and chips. N2K really just defines a set of messages that are specific to marine applications.
You are correct, but the J1939 standard that NMEA2000 is based on allows for interruptions by blind senders that actually do write over data on the bus without checking for silence. This allows simple circuits that don't even contain a CPU to transmit on the bus, and is used for things like fuel senders.

I'm pretty sure that NMEA2000 does not allow blind senders, but the CAN chipsets will do it and naive implementations exist. I did have a fuel sender that worked this way on my bus and it caused no end of havoc until I removed it.

NMEA2000 also supports periodic senders. For example, most GPSs send GPS coordinates ten times a second. Many senders are like this because its simpler and cheaper to build.

Finally, some protocols just have way too much data to transmit for N2K. AIS, for example, in a busy harbor, can overwhelm the N2K bus. The SIMRAD/B&G VHF/AIS receiver is naive like this--rather than parceling out its AIS data with some gaps to allow other senders, it bursts it all onto the bus, forcing everything else off, and can cause Autopilots to go offline.

Sadly, this set of circumstances makes what was supposed to be a DIY electronic bus require specialty tools, knowledge, and installers to get right.

What I would do is put everything that is Autopilot critical on one N2K bus (GPS, wind, AP, and chart-plotter), and all the "nice to haves" such as AIS, tank levels, engine data, auxiliary displays, etc. on another. Then put an N2K bridge between them. It will bridge all PGNs which won't resolve contention and bandwidth issues, but it will resolve timing and capacitance/resistance issues that some devices have. It will also prevent a "nice to have" from causing problems with a "must have" device. In a worst case scenario, you and unplug and drop all the nice-to-have devices if need be.

I true NMEA2000 multi-segment bridge with logging and fire-walling features would be great, but no such beastie exists.
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Old 14-07-2015, 02:48   #34
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Re: Starting from scratch

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From my reading that works well. The location on the spreader provides vertical separation from the VHF antenna. The only real issue is the deformed radiation pattern due to the proximity of guy wires and the mast, something that really should be tested somehow. Not sure how.
Yes, it provides excellent vertical separation but unless the boat is large the horizontal separation from the mast and rigging hinders transmit performance. NMEA recommended horizontal separation is 4 ft. And the tip of a 1/2 wave antenna can come very close or even touch the upper shroud.
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Old 14-07-2015, 15:17   #35
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Re: Starting from scratch

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Finally, some protocols just have way too much data to transmit for N2K. AIS, for example, in a busy harbor, can overwhelm the N2K bus. The SIMRAD/B&G VHF/AIS receiver is naive like this--rather than parceling out its AIS data with some gaps to allow other senders, it bursts it all onto the bus, forcing everything else off, and can cause Autopilots to go offline.
AIS isn't going to overwhelm the N2K bus. At 100% VDL load (the maximum possible traffic on AIS) it uses less than 38kb/s so that's not going to cause a problem for N2K operating at 256kb/s. As a comparison, NMEA 0183HS runs at 38,400 baud for AIS and the data sentences are ASCII so not as compact as N2K binary PGN's.
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Old 14-07-2015, 16:04   #36
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Re: Starting from scratch

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AIS isn't going to overwhelm the N2K bus. At 100% VDL load (the maximum possible traffic on AIS) it uses less than 38kb/s so that's not going to cause a problem for N2K operating at 256kb/s. As a comparison, NMEA 0183HS runs at 38,400 baud for AIS and the data sentences are ASCII so not as compact as N2K binary PGN's.
If only that was the way it worked in all cases. The data rate for AIS is considerably lower than N2K, but the AIS data isn't being directly bridged onto the N2K bus in real time by every AIS receiver. In the case of my receiver, the AIS receiver receives AIS data, processes it, buffers it locally, packetizes it, and then retransmits on the N2K, sadly not one report at a time. In the case of my B&G AIS receiver, it will buffer a considerable amount of AIS traffic and then burst it all at once on the N2K bus.

Look through forum threads for people complaining of autopilot dropouts on B&G/Navico systems caused by the B&G AIS transceiver doing exactly this. Turning on the AIS receiver while on autopilot is almost guaranteed to cause the autopilot to drop out because of the traffic burst from the receiver interfering with heading and rudder sensor reports from those senders.

I'm not sure if this is caused by timing issues, ballooning of the packet sizes because of NMEA2K packetization issues, or blind-sending by the AIS transmitter that overwrites messages on the bus, but its very repeatable and happens any time there are a number of AIS reports suddenly.

A naive implementation to be sure, but naive software implementations are rife in the marine electronics world, and a particular affliction in the NMEA2000 world.
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Old 14-07-2015, 16:50   #37
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Re: Starting from scratch

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If only that was the way it worked in all cases. The data rate for AIS is considerably lower than N2K, but the AIS data isn't being directly bridged onto the N2K bus in real time by every AIS receiver. In the case of my receiver, the AIS receiver receives AIS data, processes it, buffers it locally, packetizes it, and then retransmits on the N2K, sadly not one report at a time. In the case of my B&G AIS receiver, it will buffer a considerable amount of AIS traffic and then burst it all at once on the N2K bus.

Look through forum threads for people complaining of autopilot dropouts on B&G/Navico systems caused by the B&G AIS transceiver doing exactly this. Turning on the AIS receiver while on autopilot is almost guaranteed to cause the autopilot to drop out because of the traffic burst from the receiver interfering with heading and rudder sensor reports from those senders.
I can't speak to their implementation but are you sure this is what is happening? They would need to buffer up a lot of data (many seconds).

Quote:
I'm not sure if this is caused by timing issues, ballooning of the packet sizes because of NMEA2K packetization issues, or blind-sending by the AIS transmitter that overwrites messages on the bus, but its very repeatable and happens any time there are a number of AIS reports suddenly.
I'm not sure what you mean by ballooning of packet sizes. A quick look at a Class A Position Report shows N2K representation including overhead uses 286 bits and the 0183 representation including overhead uses 408 bits.

N2K devices listen on the bus and don't send blindly.
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Old 14-07-2015, 22:25   #38
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Re: Starting from scratch

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I can't speak to their implementation but are you sure this is what is happening? They would need to buffer up a lot of data (many seconds).



I'm not sure what you mean by ballooning of packet sizes. A quick look at a Class A Position Report shows N2K representation including overhead uses 286 bits and the 0183 representation including overhead uses 408 bits.

N2K devices listen on the bus and don't send blindly.
No, this is my suspicion based on troubleshooting without access to NMEA2000 specific tools. What I know is that my AIS receiver floods the bus within seconds of the AIS receive function being enabled, and issues with the autopilot and senders dropping offline occur. If the receiver (powered on and connected the entire time, so not a physical issue with the bus) is disabled in soft configuration, these problems never occur. So I am making assumptions based on troubleshooting and my lay understanding of these protocols.

NMEA includes some large/extended packet data types, and I assumed that the sender must be using those in order to cause the broadcast storm that it does. Frames in NMEA-2000 have about 32 bits of overhead per 64 bits transmitted, but that's still not enough to account for a 286 bit message flooding the bus, there would be no reason to use anything other than fast-packet, which wouldn't load the bus.

So I don't know, I'm just guessing. But I do know my AIS transmitter floods the bus and, and I know other folks with the same fit-out have the same problem.

If you've got any other insight as to why this might be happening, I'd be happy to do the troubleshooting. I suppose I could spend money on an NMEA2K data logger, but that would cost more than another AIS receiver.
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Old 14-07-2015, 22:36   #39
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Re: Starting from scratch

I had a Navico/Simrad NAIS400 for a while and it was problematic too. My MFD (A Simrad NSO at the time) would report Fast Packet Errors. My guess was that the NASI400 was violating the Fast Packet Protocol, but that was just a guess. Perhaps that's what's happening with yours. Does you MFD have a Diagnostics page for N2K? It might report that error.

I now have a Class A transceiver that feeds the N2K bus via a Furuno 0183 to N2K converter. I've been tied up near Fisherman's Terminal in Seattle with probably 100 targets on the screen, and never seen any N2K errors. Bus loading is in the mid teens %. When working correctly, there is plenty of bandwidth.
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Old 15-07-2015, 20:17   #40
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Re: Starting from scratch

So tell me about VHF with AIS and all the "ship's info" programmed into it. I am closing on a boat in the very near future. I would like to buy my VHF fixed and hand held, but if I understand correctly, the "ships info" has to be programmed in by the vendor before shipping. Obviously I don't own the boat yet, but once I do I would like to have my stuff in hand.

Further I am going to rename the boat so what's up with that part of the process?

Any experience with this? Modern "high end" vhf radios with AIS, programming the "ships info". And how hard to change once renamed?
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Old 15-07-2015, 21:31   #41
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Re: Starting from scratch

Your vessel info is programmed into the AIS not the VHF. However, both units do share the same MMSI number. You can buy a VHF without having your MMSI yet, but if you are located in the US you cannot buy an AIS without already having your MMSI number assigned. It will then come to you pre-programmed with your MMSI and other boat details.

You can change the name of the boat but cannot change the MMSI without going back to the dealer or installer.
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Old 15-07-2015, 22:01   #42
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Re: Starting from scratch

Yes, but the VHF has AIS built-in. It has an emergency button to press which xmits all that stuff. Thus (I would think) it would need all that stuff programmed in. Even my prospective HH VHF has one of these buttons. With GPS and everything.
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Old 15-07-2015, 22:22   #43
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Re: Starting from scratch

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Yes, but the VHF has AIS built-in. It has an emergency button to press which xmits all that stuff. Thus (I would think) it would need all that stuff programmed in. Even my prospective HH VHF has one of these buttons. With GPS and everything.
The emergency button is for DSC not AIS. Depending on the model, the VHF may also have an AIS receiver built-in but that doesn't require any programming. DSC requires only your MMSI number.
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Old 16-07-2015, 05:20   #44
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Re: Starting from scratch

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The emergency button is for DSC not AIS. Depending on the model, the VHF may also have an AIS receiver built-in but that doesn't require any programming. DSC requires only your MMSI number.
OK, be patient here, I have never done this. Isn't my MMSI number issued when I register an AIS device with the coast guard? And isn't that process the "this is my boat name, length, weight etc" process?

So at the moment I don't have an MMSI number. That is my point here. I don't yet own a boat. I am in the process of getting a survey done and IF it floats, sails and motors then I am on the hook to buy it, but it is not yet mine.

The boat is 700 miles away and I have to get there and sail it home. To do that I want to buy a new VHF (with built-in AIS receiver and DSC) and new VFH HH (with built-in AIS and DIS) to take on board before I sail home. Both of these VHF radios have everything required to do the emergency "push a button and transmit your AIS info in an emergency message".

But I have never registered a boat in my name with the coast guard. I don't own a boat. Yet.
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Old 16-07-2015, 05:32   #45
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Re: Starting from scratch

Electronics threads are becoming just as amusing as anchor threads. More so, when you consider that you really don't need any of it, the way you need an anchor.

I've sailed a bunch of boats, and I always feel fine with whatever is available.

Personally, the OP lost me at "no electronics worth keeping, without mentioning if they worked or what they were. IMHO, I bet there was enough.
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