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Old 04-10-2014, 06:47   #1
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Unshielded NMEA Cable?

I bought a 100m length of 2-core shielded cable to use for SeaTalk or NMEA, and sold the excess. 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?
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Old 04-10-2014, 06:52   #2
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Re: Unshielded NMEA Cable?

Are they twisted pairs? If so, it should be alright.
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Old 04-10-2014, 08:39   #3
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Re: Unshielded NMEA Cable?

As I said, there is only one ground return, so not twisted pairs.

I guess I could add two extra ground cores, but is it necessary?
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Old 04-10-2014, 08:50   #4
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Re: Unshielded NMEA Cable?

Quote:
Originally Posted by nigelmercier View Post
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?
Run the AIS in a shielded cable; the wires at 4800 shouldn't suffer from crosstalk at that short a run.

The AV cable should also work, provided it's physically robust enough. Some of that stuff is fairly fine, and not plated so it might corrode faster.

Away from marine channels, shielded tinned stranded wire is quite cheap. Shielded pair AWG#22 would be ideal for your application, and is also useful for runs to interior LED lighting when fused at 5A or less.
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Old 04-10-2014, 09:02   #5
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Re: Unshielded NMEA Cable?

I believe that the cable supplied with my Lowrance chart plotter runs both NMEA0183 interfaces together, in the same cable. It's working fine, same setup as yours.
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Old 04-10-2014, 09:58   #6
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Re: Unshielded NMEA Cable?

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As I said, there is only one ground return, so not twisted pairs.

I guess I could add two extra ground cores, but is it necessary?
The number of 'ground returns' is not relevant to twisted pair. Twisted pair just means that each pair of 2 wires (in the group of 4) are twisted down the length of cable. If you are really just using individual wires not in a cable, then you can had twist them in pairs.
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Old 05-10-2014, 04:16   #7
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Re: Unshielded NMEA Cable?

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The number of 'ground returns' is not relevant to twisted pair. Twisted pair just means that each pair of 2 wires (in the group of 4) are twisted down the length of cable. If you are really just using individual wires not in a cable, then you can had twist them in pairs.
How can I twist them in pairs if there is only one return wire to make a pair? Surely you are not suggesting that twisting two unrelated wires constitutes a pair?
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Old 05-10-2014, 06:58   #8
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Re: Unshielded NMEA Cable?

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How can I twist them in pairs if there is only one return wire to make a pair? Surely you are not suggesting that twisting two unrelated wires constitutes a pair?
NMEA 0183 spec uses a differential pair, usually labeled something like TalkerA/B or ListenerA/B. The differential wires are the ones that are twisted. Some implementations cheap out and are single-ended.
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Old 05-10-2014, 16:43   #9
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Re: Unshielded NMEA Cable?

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NMEA 0183 spec uses a differential pair...
NMEA 0183 is a clusterfudge, there have been many specifications. The units here are clearly single ended.
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Old 05-10-2014, 17:56   #10
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Re: Unshielded NMEA Cable?

If the run is just a couple of feet, almost any wiring will do. My Standard Horizon MATRIX VHF mixes single-ended 4800 and 38400 NMEA-0183 inputs, and these share a common ground (three wires total). The signal levels are 0/+5V. These are definitely not proper balanced (differential) signals, and the voltage levels are not proper RS422 levels, but it does work with the equipment I am using. I put ferrite cores on the wires to reduce potential radio interference.

If the cable run is more than a couple feet I would want to use twisted pair, or better yet, shielded twisted pair, especially if you have a marine SSB or Ham radio on board.
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Old 05-10-2014, 19:30   #11
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Re: Unshielded NMEA Cable?

Twisted pair wiring will only reduce interference when used with a truly balanced (rs422) signal (interference will affect both wires equally, and pose no problems.) If the NMEA negative wire is connected to common ground it cannot work as intended. Use a shielded wire with shield connected to common ground at the transmitting end only.

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