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For my digital electronics project, I'm making a device which will among other things have RS-485 communication.

From what I've seen, the recommended cable type for RS-485 is twisted pair, and one of the requirements for my project is that the communication doesn't need more than one pair of wires, so the two ideas fit in more or less nicely.

The most elegant way to actually connect the cable to the PCB that occurred to me was the RJ-11 jack. Unfortunately, it's heavily associated with telephone use and all suitable telephone cables I could find here were flat. From what I've seen, it's almost impossible to actually find the twisted pair telephone cable which will nicely mate with the RJ-11 plug. So to solve that problem the next best thing to me looked like the 8P8C connector and standard network cable. The middle pair on the 8P8C will connect with the middle pain on the RJ-11 jack, should I manage to get twisted telephone cable, and all network cables that are commonly used with the 8P8C already have twisted pairs, so it looks like the twisted pair problem is solved.

One thing that bothers me is what should I do with the 3 remaining pairs in the network cable?

The communication will be simplex and each device will have it's own ground-isolated power supply. In some tests I did with the unused pairs floating the communication worked fine at the expected cable length (around 50 m in my case) and at much higher data rates than those which will be used in the final design (115200 b/s in test and 9600 b/s in the final design), so one option is to just leave the pairs floating. That seems a bit wasteful to me so the other idea that I thought of would be to use the unused pairs to make sure that the voltages between the grounds of the two devices are within the specified values. It also seems a bit wasteful to me to use 3 whole pairs for that. Unfortunately I'm out of ideas what to do with the remaining pairs.

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    \$\begingroup\$ There is nothing preventing you from putting a RJ-11 plug on a twisted pair cable. Just because you can't find these ready made, doesn't mean they aren't easily available. \$\endgroup\$ – Olin Lathrop Jun 1 '12 at 11:43
  • \$\begingroup\$ @Olin Lathrop Actually I'm not talking about ready-made cables. I can't find single twisted pair cable at any of the usual suppliers here in small enough amounts to fit my needs. \$\endgroup\$ – AndrejaKo Jun 1 '12 at 11:49
  • \$\begingroup\$ @AndrejaKo, In a few odd situations I have had to make my own, I hate it and dont suggest it. Yes, we often by 1,000 feet of cable when we only need 100. They figure we will use it all at some point. \$\endgroup\$ – Kortuk Jun 1 '12 at 11:50
  • \$\begingroup\$ @Kortukā™¦ Well in my case, the 1000 feet cable rolls were of Cat5e cable instead of what I need and this looks like one of those points at which they could be used. I mean, I could always disassemble the Cat5e cable for a single twisted pair, but from the installation point of view, it's much nicer to work with a real cable instead of just a bare twisted pair. \$\endgroup\$ – AndrejaKo Jun 1 '12 at 12:01
  • \$\begingroup\$ @AndrejaKo, we purchase insulated single twisted pairs but we have to buy it in VERY large quantities. I understand your frustration, I was mostly agreeing that it was a common issue for an at home user. \$\endgroup\$ – Kortuk Jun 1 '12 at 12:17
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You must provide a common potential reference, so you need at least one pair for simplex data and one wire for common/GND. Just because it's differential transmission doesn't mean you only need two wires.

The power supplies to the nodes should be floating, and the 0V of the floating supply should be connected to the 0V on the cable. That way the node gets referenced to the master's 0V potential. For electromagnetic compatibility reasons, you should put a 1Meg resistor and a 1kV 10nF capacitor between 0V and chassis on each node, unless your nodes are truly isolated and have no exposed metal.

There's nothing wasteful about using Cat5e cable. It is the most ubiquitous type of high-performance balanced data cable you can find. You can buy it in grocery stores, after all.

Sidebar: If your RS-485 receiver chip is isolated or hooked up to a power/data isolator directly, you could do tricks with just using one pair and locally deriving the reference voltage, but that's making things way too complicated when you have the wires available.

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  • \$\begingroup\$ Actually I did use the remaining pairs for ground in the end, but forgot to put that as an answer. \$\endgroup\$ – AndrejaKo Apr 18 '13 at 21:26
  • \$\begingroup\$ "There's nothing wasteful about using Cat5e cable". Perhaps the wasteful fact is that it needs just two pairs, so it can run nearly the double length with the same cost, and have more flexibility/less diameter cable. \$\endgroup\$ – Diego C Nascimento Dec 18 '13 at 19:53
  • \$\begingroup\$ @DiegoCNascimento Sometimes just because you can specify a cable with less conductors, won't make it more adffordable. Everyone and their mother makes Cat5e. It's literally the most ubiquitous signal cable made on the planet. The economy of scale is highly in one's favor when choosing Cat5e. \$\endgroup\$ – Reinstate Monica Feb 12 '15 at 18:37

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