Unshielded NMEA Cable?

nigelmercier

nigelmercier

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As some will recall, I bought a long length of 2-core shielded cable to use for SeaTalk or NMEA, and sold the excess at cost. I still plan to use this for the 10m runs to the cockpit.

However, I'm considering using unshielded cable for a shorter run between my chartplotter and AIS VHF. It will consist of 4 single wires bundled in a sleeve, carrying NMEA at 4800 in two directions, AIS at 38400, plus ground; length about 2m. The reason is to reduce the bulk.

The purist in me says there will be crosstalk; will there be a problem in practice?

An alternative is to use shielded AV cable: CV plus L and R. Any thoughts?

EDIT Original cable here: http://www.ybw.com/forums/showthread.php?384314-For-Sale-Tinned-Screened-Wire-for-NMEA-etc
 
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How long's the cable? I'd worry less about the shielding than the fact that instead of three twisted pairs you have three single conductors and a shared return - but that is how the supplied cables are done on a number of commercial devices, so for two or three feet I think you can be reasonably confident.

Pete
 
I've used Cat5 network cable (4 x twisted pairs) in the boat for NMEA. Not had any problems. I didn't have any handy sheilded stuff, and one of the NMEA streams is running at 38.4K for AIS.
It's readily available and not that costly.
 
pagoda said:
It's readily available and not that costly.
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As is fully tinned shielded 22AWG twisted pair cable, I currently charge it out at £1.50 per M, its not really that other stuff will not work, it often will but to give an install the best chance of working both on installation and to continue to do so when other stuff is added why not just use the stuff that the NMEA specify?
 
nigelmercier said:
As some will recall, I bought a long length of 2-core shielded cable to use for SeaTalk or NMEA, and sold the excess at cost. I still plan to use this for the 10m runs to the cockpit.

However, I'm considering using unshielded cable for a shorter run between my chartplotter and AIS VHF. It will consist of 4 single wires bundled in a sleeve, carrying NMEA at 4800 in two directions, AIS at 38400, plus ground; length about 2m. The reason is to reduce the bulk.

The purist in me says there will be crosstalk; will there be a problem in practice?

An alternative is to use shielded AV cable: CV plus L and R. Any thoughts?
Click to expand...


Assuming your NMEA is RS485;

I've read a number of test reports using CAT5 cable for DMX512. The conclusions always seem to be positive in that little difference is found in operation.

DMX512 is a lighting data standard, it runs over RS485 at 250kbaud. In a theatre the cable lengths will be 100s meters.

So I think you'll have no problems with NMEA over RS485 at 38400baud using CAT5. Remember the terminator(s) and 0V (unless the link is isolated).

For short distances (a few meters) I often make my own RS485 cables by twisting a pair of single strand wires (using a drill). I also run RS485 over ribbon cable (for a few meters). Ribbon is quite good because you get a similar benefit to twisted pair and can have a screening 0V between adjacent pairs.

I haven't read the NMEA cable specification, but if it simply calls up the EIA485 standard then it is for baud rates up to 3.5Mbaud and distances up to 1.2km.

Yes there will be some cross-talk. It will be much less than the inherent noise rejection capability of the RS485 receivers.
 
nigelmercier said:
I can't easily make twisted pairs, as I only have one return. However, I can twist the AIS with the return, and leave the others straight.
Click to expand...

You should twist the data lines as a pair (sometimes called D+ & D-). That should be one pair for AIS and another pair for the 4800baud link. Then you need a separate 0V connection which is often done with the screen. Or, you could simply make a twisted triple; I use this for my SeaTalk to YAPP extension..

If the NMEA drivers are galvaniclly isolated, then the 0V connection isn't needed.

If the NMEA drivers haven't been used correctly (sometimes one output is left open and the equivalent input is connected to 0V) then a different arrangement is needed depending on the exact set-up.
 
nigelmercier said:
I can't easily make twisted pairs, as I only have one return. However, I can twist the AIS with the return, and leave the others straight.
Click to expand...
Thats a mistake by design.
Twisted cable are tolerant against magnetic fields, and on a boat you will have high current, high magnetive fields.
A shield will protect against static field. But on a boat you have low voltage.
I can not recomend such a connection cable
- picking up magnetic field
- and not to aply the differantial aproach of the concept of the differential receivers
- picking up static field as a minor risk
Try a thinner diameter of the wiring to fit the mecanical restrictions,
Willy
 
JohnGC said:
I need more detail. I'm looking for the actual names of the individual pins on each connector (assuming they have them). I'm hoping it will be possible to determine the implementation from these.
Click to expand...

Sounds like it could be a Standard Horizon GX2100 radio - and if it's not, it'll be something else with similar connections. Here's how the manufacturer suggests wiring it to their own CP180 plotter:

ScreenShot2014-10-06at091002_zps89041f2a.png

http://www.standardhorizon.com/down...atrix AIS.pdf&FileContentType=application/pdf

This technique of one "data" wire for each of several NMEA channels, plus one "ground" wire shared between them, is quite common in my experience. Regardless of the theory and RS standards (which I admit I don't fully understand) it seems to work in practice.

Pete
 
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prv said:
Sounds like it could be a Standard Horizon GX2100 radio - and if it's not, it'll be something else with similar connections. Here's how the manufacturer suggests wiring it to their own CP180 plotter...
Click to expand...
Yes, like that, but using slightly different ports. The signals are pretty much named as I said, and are single-ended, so no scope for twisted pairs. As we have seen in a recent thread, even units that appear to be differential (such as Raymarine ST60) may in practice be a kludge.

I'm effectively putting a 2m extension lead between the units, although I may shorten the original cable on the plotter as the terminal block is close to it.
 
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Since this isn't a balanced interface, using twisted pairs or real NMEA cable will have no electrical benefit.

Noise immunity will be based on the receiver input hysteresis and slew rate limits.

I would expect almost any 4 wires to work over this distance regardless for pairing or screening.
 
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