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Let's say I have two serial signals that need to be sent down a cable. This cable features a twisted pair along with a single common wire. If I were to place each serial signal onto one of the twisted pair conductors, how bad would this be?

I realize I wouldn't gain the benefit of common noise rejection had I used the twisted pair to send a differential signal, but would I ruin the quality of each signal by using a twisted pair?

Question: Would each signal in the twisted pair induce voltages into the opposite conductor thus reducing the signal quality? Or would I be better off to have these signals NOT be in a twisted pair? I'm essentially trying to determine whether or not I can be economical with my conductors, or if I should separate them.

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    \$\begingroup\$ It depends on the nature of the signals. Their frequency, amplitude and noise tolerance. There will be a crosstalk, but sometimes you can live with it. \$\endgroup\$ – Eugene Sh. Jul 22 '16 at 14:26
  • \$\begingroup\$ Yeah that would make sense. I'm assuming it would be best to pair low frequency signals and high frequency signals as opposed to two high frequency signals? As example, I would be better off pairing a 5V DC signal with a 5V serial signal rather than pairing two 5V serial signals? \$\endgroup\$ – Izzo Jul 22 '16 at 14:33
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    \$\begingroup\$ How long is the cable? I'm sure there are simple ways to multiplex the multiple serial signals and send them over the same twisted pair. \$\endgroup\$ – pipe Jul 22 '16 at 14:35
  • \$\begingroup\$ Van you pair 5V DC and the GND? It will solve your problem. \$\endgroup\$ – Eugene Sh. Jul 22 '16 at 14:35
  • \$\begingroup\$ @pipe The cable at a maximum should only be 12". I suppose I could look into that, but I think that would just add more complexity. \$\endgroup\$ – Izzo Jul 22 '16 at 14:36
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First you say you have ONE twisted pair, and then you say "one of the twisted pairs". We will assume that you really have only ONE pair and you meant to say "one of the wires of the twisted pair".

You are correct that unless you have rather low-frequency signals and a quite short (perhaps 1m) cable, you will likely experience bad to severe "crosstalk" between the two signals because they are so effectively capacitively coupled together by the twisted geometry.

There are ways of sending two different signals over a pair including time-domain and frequency-domain multiplexing. And using a Phantom Circuit.

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  • \$\begingroup\$ I forgot the word 'conductor'. I added it. \$\endgroup\$ – Izzo Jul 22 '16 at 14:44

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