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Reuse wire for NMEA 2000

Old 12-24-2015, 05:07 AM
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Default Reuse wire for NMEA 2000

So I have a wire that runs from my salon to the hardtop, it was previously used for a remote on a JBL black box audio system. I'd like to reuse it for a NMEA 2000 run or at a minimum a wired remote for my Fusion. Using my iPhone over WiFi is getting old.

I stripped the wire back after cutting the old remote off and this is what I'm working with. For guys with more experience than me... can I just use the a field attachable connector and make a NMEA 2000 cable out of this?
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Old 12-24-2015, 06:20 AM
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Well not sure if that will work but you certainly can give it a try. Field attachable connectors most likely will not fit that cable size. I would take a NMEA2K cable and cut it in half, and use terminal strips or splice onto you exiting cable at both ends.

1.6 NMEA 2000
® Cable

The Mini, Mid and Micro cables contain five wires: One
twisted pair (red and black) for 12VDC power, one twisted
pair (blue and white) for signal and a drain wire (bare).
The following table shows the color, name, and usage for
each wire contained within the cable.
Color Name Usage
White NET-H Signal
Blue NET-L Signal
Bare SHIELD Drain
Black NET-C Ground
Red NET-S Power

Jim
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Old 12-24-2015, 09:56 AM
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Originally Posted by Lead Change View Post
... can I just use the a field attachable connector and make a NMEA 2000 cable out of this?
Those field attachable connectors are pricey. I think you can get a complete 20' drop cable for the price of one connector.

Why not get rid of the wire and use a regular NMEA 2000 cable? Like jfwireless said, you could cut & splice the NMEA 2000 cable if you needed to.
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Old 12-24-2015, 10:13 AM
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Te price of such field attachable is not that much.
App. 20$ plus shipping for pair(male&female), makes it worth to give try to re use the cable.

Mating parts you'll find here:
http://www.newark.com/hirschmann/elk...&categoryName=
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Old 12-24-2015, 10:35 AM
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Originally Posted by Detlefb View Post
Te price of such field attachable is not that much.
App. 20$ plus shipping for pair(male&female), makes it worth to give try to re use the cable.

Mating parts you'll find here:
http://www.newark.com/hirschmann/elk...&categoryName=
Thanks for that link. I had not seen prices that low, and I wouldn't have thought to look for that specific connector at Newark.

With that said, a 6-meter cable can be bought for under $30 including shipping:
Amazon Amazon
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Old 12-24-2015, 10:48 AM
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Originally Posted by yabbut View Post
Those field attachable connectors are pricey. I think you can get a complete 20' drop cable for the price of one connector.

Why not get rid of the wire and use a regular NMEA 2000 cable? Like jfwireless said, you could cut & splice the NMEA 2000 cable if you needed to.
30+ year old wood Carolina boat. Running new wire would mean dropping the headliner and not really looking to do that right now. Eventually yes, will drop it and run HDMI, NMEA, and Video from the bridge to the engine room but this is more a quick win thing.

Question though... with no internal bare on the wire I have would I just use one of the shielded strand I have or just not use it? Not sure how sensitive NMEA is to the ground.
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Old 12-24-2015, 10:50 AM
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@yabbut,
you are always welcome.
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Old 12-24-2015, 10:58 AM
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I wouldn't use it for N2K. An N2K cable needs to have an AC characteristic impedance of 50 ohms (I'm pretty sure it's 50, but it might be a different number) for proper signal integrity. If it doesn't then signaling will be compromised. It might work fine, it might work some of the time, it might not work at all, but you will never be sure. And if there are problems they will be a nightmare to track down.
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Old 12-24-2015, 11:34 AM
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A NMEA cable does not require use of connectors. Terminate your existing cable in a connector block. Buy a short NMEA-2000 drop cable, cut in half, wire to the terminal blocks.

The NMEA signal lines should be on a twisted pair, but for one drop cable, not very long, ought to work even it the pairs are not twisted. The use of twisted pairs is to improve common mode rejection for the differential input amplifier. Your in-situ cable has a braided shield; use that as the shield conductor.

The surge impedance of the cable is really irrelevant for the short distance because there is no transmission line effect. Transmission line effects occur when the distance is long compared to a wavelength at the frequency of the signal. Because the cable connects to a backbone with a 60-Ohm impedance, the surge impedance of the cable is mostly insignificant and the resistive termination of the network wire pair sets the impedance.

At least try it and see if it works. It will be much cheaper and faster to use two terminal blocks and a cut-in-half premade cable to test with. Tie the unused conductors to the shield. Also, your existing cable may have a shielded pair inside of the shielded cable. Use that for the NMEA signal.

Last edited by jhebert; 12-24-2015 at 12:30 PM.
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Old 12-24-2015, 02:18 PM
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Originally Posted by jhebert View Post
The surge impedance of the cable is really irrelevant for the short distance because there is no transmission line effect. Transmission line effects occur when the distance is long compared to a wavelength at the frequency of the signal. Because the cable connects to a backbone with a 60-Ohm impedance, the surge impedance of the cable is mostly insignificant and the resistive termination of the network wire pair sets the impedance.
I admit it's been a long time since I worked on this stuff, but there is a difference between the termination resistance which is 100% resistive, and the AC impedance of the cable. The N2K differential pair is an AC signal for all intents and purposes, and anytime it encounters a change in the line's AC impedance, say from 50 ohms to 60 ohms, part of the signal continues forward, and part of it gets reflected back in the other direction. The split in the signal is proportional to the difference in impedances. This creates two potential problems. 1) the forward signal is now attenuated and may be out of spec when it reaches the furthest device. 2) the reflected signal now creates a potential false signal that might be detected by devices on the network.

I'd say chances are 99% that if you just wire it up any old way it will at least appear to work just fine. But do you want something that appears to work, or something that you can count on working across all normal conditions?
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Old 12-24-2015, 03:02 PM
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Originally Posted by Lead Change View Post
30+ year old wood Carolina boat. Running new wire would mean dropping the headliner and not really looking to do that right now. Eventually yes, will drop it and run HDMI, NMEA, and Video from the bridge to the engine room but this is more a quick win thing.

Question though... with no internal bare on the wire I have would I just use one of the shielded strand I have or just not use it? Not sure how sensitive NMEA is to the ground.
Twist the shielded strand on your cable into an single wire and connect to the shield drain on the NEMA 2K cable. Connect your black wire to the NMEA 2K black ground wire.

Give it a try, might just work.

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Old 12-24-2015, 03:43 PM
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Originally Posted by twistedtree View Post
I admit it's been a long time since I worked on this stuff, but there is a difference between the termination resistance which is 100% resistive, and the AC impedance of the cable. The N2K differential pair is an AC signal for all intents and purposes, and anytime it encounters a change in the line's AC impedance, say from 50 ohms to 60 ohms, part of the signal continues forward, and part of it gets reflected back in the other direction. The split in the signal is proportional to the difference in impedances. This creates two potential problems. 1) the forward signal is now attenuated and may be out of spec when it reaches the furthest device. 2) the reflected signal now creates a potential false signal that might be detected by devices on the network.

I'd say chances are 99% that if you just wire it up any old way it will at least appear to work just fine. But do you want something that appears to work, or something that you can count on working across all normal conditions?
I am wondering if the OP used his cable as a 'backbone extension' instead of a dropper and fitted a tee and the 120 ohm terminator at the far end giving 60 ohms impedance, would this not solve any impedance mismatches you suspect by using none standard cable,what do think? Keep in mind we are not talking 50 metre cable runs up yacht masts for example and at 256 kbps would have wavelength of 1.2 kilometers.
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Old 12-24-2015, 05:10 PM
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I believe better as a drop than backbone. No need to upset the backbone, that would have more impact than on a stub drop.

This most likely work if not too long. We are not on the lab, just trying to get something to work until the OP has time to run a new cable. This is a simple network not running at super high speeds with optical isolators at both ends. Not being a twisted pair cable should not affect much if there is no significant ingress along this cable route.

If the OP sees cable pairs close together at both ends he can try to use them as twisted pairs. Try and isolate the two pairs in the bundle as far apart as possible. We do have a shielded cable here, that will help. I would tie all conductors not being used in the bundle to the shield drain as well.

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Old 12-24-2015, 05:22 PM
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Originally Posted by isitstuffed View Post
I am wondering if the OP used his cable as a 'backbone extension' instead of a dropper and fitted a tee and the 120 ohm terminator at the far end giving 60 ohms impedance, would this not solve any impedance mismatches you suspect by using none standard cable,what do think? Keep in mind we are not talking 50 metre cable runs up yacht masts for example and at 256 kbps would have wavelength of 1.2 kilometers.
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.
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Old 12-24-2015, 10:32 PM
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Originally Posted by Detlefb View Post
@yabbut,
you are always welcome.
Baltic Fisher appears to be the name of your vessel...wonder about make/model, and whether it has a US distributor?

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Old 12-25-2015, 04:50 AM
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Pretty interesting read from some pretty educated guys... appreciate all of this. Electronics aren't my thing (I prefer motors and carpentry) but I've done enough DYI projects not to be scared of it. However when we get into impedance and ohms my eyes glaze over.

Talked to a local guy who said if everything goes smooth he can re-stretch my headline for a couple hundred. Think I'm going to order a few field connectors but also go ahead and make the long pull from to bottom so next question...

41' convertible. will have potential for NMEA devices from top to bottom. Certainly at the helm, stereo in the salon, but will (at some point) have sending units on the tanks or other monitors in the ER. The 12v power for the NMEA network is in the helm on the bridge. Really want to avoid moving it since the 12v breaker panel is in the helm. She's a 24v boat with a small converter for the electronics and stereo so limited options for 12v.

Do you think I can make a run from top to bottom and be ok on the power or do need to find 12v somewhere else at the mid point?

Also... I never realized there is micro and mini cables... since I'm more or less starting from scratch is there a benefit to either or?
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Old 12-25-2015, 07:06 AM
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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 will admit, if the difference in surge impedance is to an impedance of 0-Ohms (a short) or an impedance of infinite-Ohms (an open circuit), there will be some effect.

Recall that my recommendation was to try using the cable. I believe it will work. I do not believe there is anything to be gained by abandoning use of the cable without testing on the basis that someone has decided that there may exist a slight impedance discontinuity for 0.0014 wavelength in the cable. So far we have not been provided with any basis to know the surge impedance of the conductors of the in-situ cable and to assess how much difference there might be. So there is really no basis to the claim they will cause a problem. Their surge impedance may be quite close to the recommended cable.

The loss of the twisted pair may affect the common mode rejection, but if the cable is well shielded, this should be overcome by the shielding.

Last edited by jhebert; 12-25-2015 at 07:17 AM.
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Old 12-25-2015, 07:20 AM
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Originally Posted by Lead Change View Post
...I never realized there is micro and mini cables... since I'm more or less starting from scratch is there a benefit to either or?
The heavier gauge cable can carry more current in its power conductors. This may be beneficial in networks with very long backbone length and many devices requiring power from the network. On the typical small boat installation, the heavier cable is usually not necessary.

For more information, refer to NMEA's own presentation at

http://www.nmea.org/Assets/2012%20ib...stallation.pdf

Note that cable types are specified as LIGHT, MID, and HEAVY, not as mini or micro.

Cable-end connectors are described as MINI or MICRO.

The usual wiring practice is to use MICRO connectors and LIGHT cable. An option is to use MICRO connectors and MID cable.
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Old 12-25-2015, 01:26 PM
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Originally Posted by Lead Change View Post
Talked to a local guy who said if everything goes smooth he can re-stretch my headline for a couple hundred. Think I'm going to order a few field connectors but also go ahead and make the long pull from to bottom so next question...
I was thinking in terms of removable ceiling panels, but it sounds like your ceiling access is much more complex, so I understand your hesitancy to pull new cable. Pulling wires is my least favorite part of electric and electronic projects.

Maretron has a good cabling guide on their web site (maretron.com). It talks about the different types of cable, connectors, cable length rules, termination, etc. They also have a good tool called N2KBuilder where you can enter in the details about your system including cable lengths, types, Tee connectors, drop cables, and all the connected devices. You can create your own devices easily with the correct LEN number which is how much power each device draws. The program will show you any places where you are breaking rules, and will calculate the power voltage drop from your power tap to the various devices, and tell you if you have fallen out of spec. It's way easier to model and check it in advance than to rewire and fix problem later.

As to power and voltage drops, on smaller networks it's rarely a problem, but you never know for sure unless you check with something like N2KBuilder. I built one smallish system with a weather instrument at the very end of the bus plus a display almost to the end. And I used the micro cable (the small stuff that is used for drop cables) for the backbone. The weather instrument and display both where high LEN devices and the calculated voltage drop by the time you reached them was cutting it close. As I recall, one minor adjustment in the location of the power tap added a bunch of extra margin back into the system.
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Old 12-25-2015, 01:30 PM
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We are getting too complex for a simple decision regarding the use of a spare cable. All of the discussion on cable impedance which is AC resistance refers to AC but I question that, as we are talking about digital data, square waves not sinusoidal radio waves. The only concerns are rise and fall times and amplitude losses. Enough already... To use the proper cable is of course the best option, however the OP wants to second a none approved cable. Using it as a dropper will probably work but only for one device. If he does that, and it then works it is a simple job to reconfigure as a backbone extension therefor allowing multiple devices in a remote location, which after all is how the system is supposed to be installed. The cable only has to do 2 things, supply power to the CANbus chips in the devices and convert it to a digital signal via a phototransistor. There is no physical electrical connection for signals from devices. There is the ability however to use this network power for low power devices as we know and measured in 50mA network loads. As the OP wants to install other devices using the cable in question as a backbone, will enable it to be cut and 'teed' into for these other devices.
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