RS-485 (Modbus) Cable

For an upcoming project, the GC has chosen this wire:

https://www.mayerelectric.com/itemDetailFilterPH.action?codeId=1189529

There will be three nodes, 700 feet will be the longest run. I don't like that cable, not sure it will work. I thought it was at least an unshielded twisted pair, but it doesn't even appear to be twisted. I can ask him to change it, but I want to be sure before I cause an unneeded expense for him.

Any thoughts?

Thanks,
Paul

It could work, but I wouldn't risk it.
Repulling a 700ft run would cost alot more than getting the proper cable.

It's hard to prove a negative; I can't say for sure that an untwisted pair will not work, but I can say that a twisted, shielded pair is recommended and has a far higher probability of working reliably.


The EIA-485 standard does not, to my knowledge, actually recommend a wire type. It does recommend 120 ohm characteristic impedance.

I would bring him a couple of vendor's recommended product sheets for RS485:

Belden's RS-485 cables


General Cable's recommendations for RS485

Whether it works or not depends on a long list of environment variables you don't know and the unpredictable ways in which they will interact. I've heard of RS-232 serial runs of 100 ft run in the same conduit as power working almost flawlessly (whenever a motor started, the surge would sometimes corrupt the data during transmission). That doesn't mean it's a good idea, of course.

I guess the selling point could be, without running shielded cable you're risking mysterious, hard-to-troubleshoot errors cropping up. The added cost of a shielded cable is well worth it if it avoids risking the expensive nightmare troubleshooting a network issue can be.

I already fired off an email to tell him I didn't like it and can't be sure it will work. I found an article to support my case and recommended Belden 3106A.

Thanks for the feedback.

As it turned out, he has not used any of that "speaker cable" yet and thinks he can return it.

Here is one of the links I sent that may prove useful for future readers of this thread:
http://www.chipkin.com/rs485-cables-why-you-need-3-wires-for-2-two-wire-rs485/