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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.

Izzo
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    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. – Eugene Sh. Jul 22 '16 at 14:26
  • 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? – Izzo Jul 22 '16 at 14:33
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    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. – pipe Jul 22 '16 at 14:35
  • Van you pair 5V DC and the GND? It will solve your problem. – Eugene Sh. Jul 22 '16 at 14:35
  • @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. – Izzo Jul 22 '16 at 14:36
  • There are encoding methods especially suited for this if you actually run into problems. Twisted pair or not depends on several HF factors. Ethernet CAT6 cable has a different amount of twist for different pairs to reduce crosstalk. Is this something you can exploit? See http://electronics.stackexchange.com/questions/20023/why-does-usb-have-4-lines-instead-of-3 – winny Jul 22 '16 at 14:37
  • @pipe Simple? It will require some extras on the both ends anyway. – Eugene Sh. Jul 22 '16 at 14:37
  • @EugeneSh. I could do that. However, I've got a few different signals like UART, SPI, I2C, 5V analog signal, 5V, 3.3V, GND. I'm trying to determine the best pairs. – Izzo Jul 22 '16 at 14:37
  • Best pairs would be differential signals. Power pairs are naturally differential. – Eugene Sh. Jul 22 '16 at 14:39
  • @EugeneSh. Absolutely extras. But I was under the assumption that the cable was at least 10 metres or something, meaning that it's cheaper to put two adapters on the ends than it is to replace the cable. – pipe Jul 22 '16 at 14:42
  • **1)** What are the frequencies and amplitudes (voltage and current) of the signals? **2)** How many twists per inch? **3)** What are the diameters of the conductors? **4)** What material is the insulation made from and what's its diameter? **5)** Is this commercial cable? If so, who's the manufacturer and what's the cable's part number? **6)** is the cable shielded? – EM Fields Jul 22 '16 at 15:05
  • @EMFields Hi EMFields, thanks for the response. I'm currently looking at transmitting SPI, I2C, UART, via a single HDMI cable. I realize there are concerns regarding this, however, the typical usage is just a direct plug in with HDMI receptacle and plug (i.e. no cable), I'm simply looking to see how feasible it would be to add a cable. – Izzo Jul 22 '16 at 15:12
  • Is this question at all different to this one you raised a few days ago: http://electronics.stackexchange.com/questions/246161/what-are-the-effects-of-single-ended-signals-on-twisted-pair-conductors – Andy aka Jul 22 '16 at 16:36
  • @Andyaka Yes, that question was inspired by a completely separate application where I was implementing high speed serial. I was curious of the effects of a "hanging" or unused twisted pair conductor. – Izzo Jul 22 '16 at 16:44
  • The inspiration may be different but the question remains largely the same. – Andy aka Jul 22 '16 at 16:47
  • Yteague, you wrote: "Hi EMFields, thanks for the response. I'm currently looking at transmitting SPI, I2C, UART, via a single HDMI cable." If you'd have stated that up front it would certainly have saved everyone a lot of time and, I suspect, aggravation. Go [**here**](https://www.google.com/search?q=hdmi+cable+construction&tbm=isch&tbo=u&source=univ&sa=X&ved=0ahUKEwizwaPmyYfOAhUozIMKHRMkCDYQ7AkIOQ&biw=1280&bih=937) for a clue. – EM Fields Jul 22 '16 at 17:19
  • @EMFields This question was posed more general such that future users could reference the question including those who aren't working with HDMI. I was attempting to get some general responses regarding past experiences on signal coupling in doing what I described. – Izzo Jul 22 '16 at 18:35
  • If you'e working with HDMI, then your question should include that information in order that a proper solution be found for **YOU** by eliminating the inevitable confusion caused by asking a generic question with dubious relevance later. Now, since you say you want to use an HDMI cable, can you edit your question with a schematic showing how you plan to do signal routing and timing diagrams showing signal edge rise and fall times, frequencies and amplitudes, please? – EM Fields Jul 22 '16 at 19:30

1 Answers1

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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.

Richard Crowley
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