RS-232 using shield for ground connection?

elmatador · Sep 21, 2011

Hi Guys,

The electricians accidentally pulled a two conductor cable for the RS-232 communications. The seals are poured. Is it okay if I use the shield wire for the GND pin?

I know this is not the proper way and you should have a three conductor cable with shield. The distance is about 70ft between buildings.

Let me know what you guys think?

bce123 · Sep 21, 2011

will your plc vendor accidentally warranty your product this way? do you need the sheild for noise? did you pay your electrican by the hour or by the job?If by the job have him pull new wire. did you supply the cable or did he? if he did have him buy new wire.if you supplied it buy new wire and pay him to repull it. chico/seals can be chiped out and vacumed out you have the chance to do it right why would you want anything less?
Just my thoughts, Steve

DamianInRochester · Sep 21, 2011

The rule of thumb with RS 232 is 50ft. You can get away with longer lengths, but only by doing things right, not wrong. The problem with using the shield as a conductor is twofold. The first is obviously you loose your shield "as a shield". The other is that a sheild is designed to "wick" the noise away, thus the signal you put on the shield will actually absorb more noise than the other regular conductors.

I'd make them redo it.

iant · Sep 22, 2011

I agree totally with Damian
Comm Noise is a cause of intermittent faults
Prevention is the best method

I am an electrician but would not make that mistake.

Liam Moran · Sep 22, 2011

The shield on a comms cable should only be used as a noise protection shield, and ideally should be grounded at only 1 end to prevent it to be used as a ground loop, particularly between buildings.
At 70 feet too you may have to use a reduced baud rate.

MichaelG · Sep 22, 2011

If the RS232 comms is half duplex then you could get some rs232/rs485 converters and wire them in 2 wire mode - Bonus is rs485 permits a longer cable run

If staying RS232 then redo cable just like Damian said

iant · Sep 22, 2011

Michael - Even with two wire there is normally still a shield required

I is a noob · Sep 22, 2011

I would go with 485 if it is possible, you could try it and cross your fingers.

danw · Sep 22, 2011

>The seals are poured.

Sounds like a hazardous area. If the wire runs in steel conduit you have some shielding from the conduit.

What do you stand to lose by trying it as is? All the warnings are true, but hey, if it works, it works.

bce123 · Sep 22, 2011

What do you stand to lose by trying it as is?

most likley just pride in your work!

elmatador · Sep 22, 2011

I appreciate all the feedback and thoughts on this. I'd rather have it done right like others have mentioned than to rely on some shoddy wiring. I got them to bust the seals and they will be pulling new teck. They were not happy, but hey at least now I know it will be solid.

If the distance issue does become a problem then I will get some RS-232/RS-485 converters to handle the issue.

Thanks guys.

leitmotif · Sep 22, 2011

If it had been my call and especially so with seals I would have pulled in cable with at least two or 3 spare conductors. That way IAW Murphy Law I will never have to fool with it again.

War story while on submarine we developed a ground in Trim Pump control wiring. Opened it all up and found a spare in the cable we could use. Repulling the cable would have been big job just sorting it out of the cable maze AND testing and certifying two pressure bulkheads.

Moral of story - pull in spares.
Dan Bentler

iant · Sep 22, 2011

here here - I agree fully Dan

RS-232 using shield for ground connection?

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