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I am hunting around for a cable with twisted-shielded pair conductors with the correct differential characteristic impedance for RS-485 (120 ohm)

Most of the cable spec sheets for cables with twisted shielded pair just say "Characteristic Impedance = ... " (here is an example: http://www.alphawire.com/Home/Products/Cable/Alpha-Essentials/Communication-and-Control-Cable/6365?device=pdf)

Is this referring to the differential impedance of the two conductors or the impedance of a single conductor?

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  • \$\begingroup\$ A single conductor can't have a differential impedance. Differential implies a difference between two things. \$\endgroup\$ – Tom Carpenter Feb 20 '18 at 23:31
  • \$\begingroup\$ The spec sheet I am looking at is for a twisted-shielded pair, not a single conductor. I will modify the question to make that more clear. \$\endgroup\$ – user2913869 Feb 20 '18 at 23:33
  • \$\begingroup\$ It is referring to the transmission line impedance. A twisted pair is a balanced line. Google transmission line for more information. It takes into account that both wires are the transmission line. 120 ohms is pretty common for twisted pairs. I'm surprised that your link specs 75 ohms. \$\endgroup\$ – lakeweb Feb 20 '18 at 23:42
  • \$\begingroup\$ @lakeweb, differential impedance of a twisted pair is a function of ratio between conductor diameter and insulation jacket thickness/dielectric constant. For example, Belden makes cables with 52-Ohm characteristic impedance, and even 36-Ohm., catalog.belden.com/techdata/EN/82723_techdata.pdf \$\endgroup\$ – Ale..chenski Feb 21 '18 at 3:26
  • \$\begingroup\$ HI @Ali Chen, Thanks, yes, I know. I had cat5 in my head and now that I've checked, I see the nominal spec is 100 ohms. For his app, all other things being perfect the SWR from 75 ohms to 120 ohms is 1.6. So only ~5% loss. \$\endgroup\$ – lakeweb Feb 21 '18 at 16:20
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You don't want the 6365 Alphawire for the connection: while there are 5 twisted pairs, the pairs are not shielded individually. You might have unwanted interference. Better use Belden 9842, 120-Ohms. But it has common shield. If 1-pair cable, it has 120 Ohms in one shield..

For RS-422 (100 Ohm) there is Belden 8162, with individually-shielded pairs.

I am sure if you search deeper, you will find 120-Ohm individually-shielded multi-conductor cable.

CORRECTION: since the RS-485 is supposed to be completely balanced, the cross-talk is very diminished, which allows to use non-individually shielded twisted pairs. The classic cable for two full-duplex links is Belden 3109A, pictured in TI guide.

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