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Reuse wire for NMEA 2000

Old 12-25-2015, 01:56 PM
  #21  
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Originally Posted by jhebert View Post
The AC signal in NMEA-2000 is at a frequency of 230-kHz. The wavelength is thus 4,280-feet. A drop cable whose length is 6-feet is roughly an AC wavelength distance of 0.0014-wavelength.

To consider the effect on a transmission line of a discontinuity in impedance of 0.0014-wavelength, we now assess this at a radio frequency with which, as boaters, we are familiar, 156-MHz. At 156-MHz a distance of 0.0014-wavelength would be about 0.050-inch.

Now, if there is anyone who thinks that some serious harm occurs if a radio signal at 156-MHz travels through a portion of its circuit for a distance of 0.050-inch without being in a 50-Ohm transmission line while connected to a resistive 50-Ohm termination, then those same people will be concerned if a NMEA-2000 signal travels over 6-feet of wire that is not the same surge impedance as the rest of the circuit.
I think we must be talking about different things. I agree that the length of the "different" cable is not very important. The magnitude of the reflected signal depends only on the impedance difference at the transition point, not how long the cables are on either side. When and how the reflected signal coincides with the original signal, whether they are additive or subtractive, or appear as a completely new signal, depends very much on the cable lengths and frequency of the data. But we really are sinking into electronic minutia at this point. Here's a good article is anyone actually cares: https://en.wikipedia.org/wiki/Reflec...nducting_lines
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Old 12-25-2015, 02:35 PM
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Originally Posted by twistedtree View Post
We're getting pretty deep into electronics here, so just skip over if you don't care. The data signals on an N2K cable are constantly moving, much like a radio signal from your VHF. These signals react differently with wires compared to DC power like from your battery. If you measure the resistance of an N2K cable with an ohm meter, it will show a very low value. But if you try to send different frequency signals down it, it may pose more resistance, and might even look like a brick wall. So you need to pick the cable so it appears to the N2K data signals just the same as the 60 ohm termination resistor at the very end. The term "impedance" describes the more complex "resistance" encountered by higher frequency signals. What determines the impedance of a cable are things like the conductor wire, but also the type of insulation, its thickness, the spacing between the wires, each wires proximity to the others, and probably a whole lot of other stuff that I don't know about.

When you get VHF antenna wire, you need to use 50 ohm cable. If you measure that wire with a multi meter, it won't measure 50 ohms because it's measuring it's DC resistance. But at VHF frequencies, that cable looks like 50 ohms and will prevent reflections of your transmit signal.

The same is true for N2K, but of course it's operating at a different frequency. But the cable is "tuned" to match the termination resistors, at least at the frequency of the signals. This prevents/minimized reflections and signal attenuation. The same is true of the N2K Tees and other connectors.

Like I said early on, if you ignore this and just use whatever cable for your backbone or some part of your system, it will most likely work just fine, or at least appear to. But you will have no idea what sort of reflections and attenuation is going on in your network, and whether it's operating within a safe range or close to the edge where it's prone to generate random errors.

For me personally, I find it hard enough to get all this stuff to work properly with an electrically correct network, so the last thing I want to do is introduce a questionable network. And if you have having problems with your network, the first thing all the vendors will do is blame it on your backbone. Your best defense in this case is to have a properly built network.
Your reference to resistance when talking about an RF circuit not DC is really reactance. As the original twisted pair was open wire, I really do not think we have much of an issue here. The length of cable and the capacitance between the conductors is the real story .

If it works I really think it will be OK, I would not have all those worries about have about standing waves and maybe not working 100% of the time. It will work or it will not. Just try it and let us know how it works.

Jim
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Old 12-26-2015, 02:37 AM
  #23  
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[QUOTE=Karl in NY;8548625]Baltic Fisher appears to be the name of your vessel...wonder about make/model, and whether it has a US distributor?

Hello Karl, sry for my bit late reply, Christmas is blocking me particular

Yes you're right Baltic Fisher is the vessel name.

The boat is a Jeanneau Merry Fisher 625 ,French design, made in Poland.
It's made 2009.
The successor models are Merry Fisher 645 and now Merry Fisher 695.

Look's like there are some importer to US market. Please checkout at: http://www.jeanneau.fr/
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Old 12-26-2015, 06:26 AM
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Originally Posted by twistedtree View Post
I think we must be talking about different things...
Not really. I understand transmission theory. In this case the frequency is just very low and the length of the transmission line is just very very short.

Also--IMPORTANT--the rules for maximum length of a drop cable still apply. If you use your in-situ cable as a drop cable to the device from the network backbone, it cannot be longer than 6-meters or 19.7-feet.

Last edited by jhebert; 12-26-2015 at 06:37 AM.
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Old 12-26-2015, 06:57 AM
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Originally Posted by jhebert View Post
Not really. The input impedance of any of the transceivers of the devices connected to the bus is much higher than the terminating impedance of the bus. The devices bridge the bus, they don't load it. If every device on the bus had an impedance matched to the bus cable terminating impedance, the bus impedance would be lowered each time a device was added.
Somehow we are talking across each other. In one sentence you seem to understand all this, but then in the next something gets confused. In the above you are confusing the whole concept of complex impedance (real plus imaginary components) which is what every cable, connector, and device presents as its input impedance, and the 100% real impedance (purely resistive) of the terminators. I think the same confusion came into play earlier when you were talking about how long a distance of cable the signal has to travel across at some non-standard characteristic impedance.

Every device on the network has a complex input impedance of 50 ohms, So does every cable, and every connector. And so does the terminator resistor. The difference is that the terminator resistor is (almost) purely real, where all the other gear is (almost) purely imaginary, using the electrical engineering definitions of "real" and "imaginary".

It's worth a look at the wiki article that I posted earlier. It's heavy on the math, but the text helps explain the notion behind a transmission line, characteristic impedance, and a matching termination resistor to prevent reflections and make the transmission line appear to be infinitely long.
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Old 12-26-2015, 07:12 AM
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Originally Posted by jhebert View Post
Not really. I understand transmission theory. In this case the frequency is just very low and the length of the transmission line is just very very short.

Also--IMPORTANT--the rules for maximum length of a drop cable still apply. If you use your in-situ cable as a drop cable to the device from the network backbone, it cannot be longer than 6-meters or 19.7-feet.
I think the difference here is that you are talking about signal loss per foot of cable, and I'm talking about signal reflections at impedance discontinuities in the cable. I agree that signal loss per foot of cable is not an issue here, and that using a cable with a different characteristic impedance won't materially change the signal loss per foot of cable. But I'll emphasize the "per foot of cable run" part of the statement above, because there is still signal loss due to reflection. And the reflection doesn't happen gradually over each foot of cable, it happens exactly where the cable's characteristic impedance changes, i.e. at the transition to the non-compliant cable.
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Old 12-26-2015, 04:06 PM
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From experience I spent a number of years in a classroom environment studying to be an electronics engineer and spent literately hundreds of hours working on formula's expressing the concepts discussed above and at the end of the day never needed to use any of it in a service / repair environment.
In the majority of cases I go by common sense, gut instinct, theory of operation, reading the manuals and taking measurements using the appropriate devices.
I see one problem in this discussion is the inability to distinguish that we are talking about data transmission like 10Base-T, CANbus stuff with differential pair noise cancellation, and the subject has drifted to radio wave propagation through a transmission line (or THT thread) of infinite length... hopefully...and relating to radio co-ax stubs and SWVR.
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Old 12-26-2015, 06:35 PM
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The signalling on the CAN bus is a square wave not a sine wave so minimum the third and fift harmonics of the transmission frequency have to be considered so the interesting bandwidth is not 250kHz, but minimum 1.25MHz.

The best way to check the quality of a NMEA 2000 (CAN bus) physical layer is to use a eye diagram. The example below is from a 500 kbit/s automotive application. There should be no transitions in the gray mask.



I would not even consider using cabling with incorrect impedance for my NMEA 2000 network unless I had the possibility to verify the opening of the eye diagram.

Here are measurements I've done on my NMEA 2000 network, the first with a faulty device and the second after the faulty device was replaced.(the reason why my images are upside down compared to the one above is that the example is showing the CAN bus in dominant-bit-low mode and I've used dominant-bit-high.)



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Old 12-27-2015, 07:42 AM
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Originally Posted by isitstuffed View Post
From experience I spent a number of years in a classroom environment studying to be an electronics engineer and spent literately hundreds of hours working on formula's expressing the concepts discussed above and at the end of the day never needed to use any of it in a service / repair environment.
In the majority of cases I go by common sense, gut instinct, theory of operation, reading the manuals and taking measurements using the appropriate devices.
I see one problem in this discussion is the inability to distinguish that we are talking about data transmission like 10Base-T, CANbus stuff with differential pair noise cancellation, and the subject has drifted to radio wave propagation through a transmission line (or THT thread) of infinite length... hopefully...and relating to radio co-ax stubs and SWVR.
This is true but remember a square wave is made up of an infinite number of sine waves, the higher the frequency response relative the center frequency in the transmission system determines just how square or rounded the wave becomes as it propagates down the transmission system. At the end of the transmission system the receiver takes what is left of the square wave and reproduces a square wave again If possible. Yes all other kinds of things can happen, but at this relatively low switching frequency I really think this has a chance to work. I have done similar things many times.

Once again, this has gotten way overboard for what the OP asked, he is not a EE. He just wanted to know if this might work to avoid having to run a new cable in a difficult cable run. Yes it is worth a try, if it works awesome, if not he has to run a new cable. Cost to try maybe $30 for a new cable, but I am sure some of us have old cables we can send to him if he wants to try it. Cost for an engineering evaluation to see fi this will work PRICELESS

remember KISS.

Jim
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Old 12-27-2015, 01:52 PM
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Originally Posted by jfwireless View Post
This is true but remember a square wave is made up of an infinite number of sine waves, the higher the frequency response relative the center frequency in the transmission system determines just how square or rounded the wave becomes as it propagates down the transmission system. At the end of the transmission system the receiver takes what is left of the square wave and reproduces a square wave again If possible. Yes all other kinds of things can happen, but at this relatively low switching frequency I really think this has a chance to work. I have done similar things many times.

Once again, this has gotten way overboard for what the OP asked, he is not a EE. He just wanted to know if this might work to avoid having to run a new cable in a difficult cable run. Yes it is worth a try, if it works awesome, if not he has to run a new cable. Cost to try maybe $30 for a new cable, but I am sure some of us have old cables we can send to him if he wants to try it. Cost for an engineering evaluation to see fi this will work PRICELESS

remember KISS.

Jim
Yes that's all true. I remember connecting up a whole heap of signal generators at College trying to make a square wave from sine waves Abbors comment re 5th Harmonic comes close. There are some NMEA2k products that come with the terminators already fitted and these items, for example, mast head units on yachts which are connected to the network as a backbone extension because its impractical to put tees and terminators on top of the mast. Simrad complicated things with their Simnet system using inbuilt terminators in their network power supply cables identified with a red disc that can't be seen once connected. The cable system eventually settled on by the NMEA Org required cable adaptors to bring their products into the new standard. I have a network tee hard wired to my multimeter so I can tap into a network and measure it's voltage and confirm 120ohms parallel resistance and by removing each terminator confirm their value too and determine if any device has a terminator. Can determine how many power sources are supplying the network and what those sources are. Such a simple faultfinding tool saves hours stuffing around cutting cable ties and chasing the dream. So as you say use the cable and try it. The system is very forgiving which is why it was selected to be a comms standard in the first place and the cable is after all a data cable not a trailer cable.
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