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There are 4 twisted pairs in a Cat 5 networking cable, creating 4 bundles. Are these bundles twisted themselves, e.g. around cable center or pairwise?

I'm trying to explain noise immunity of different cable setups for signal transmission in an industrial automation equipment. The signal consists of 50ms, 24V, 8mA pulses a few times per second between optoisolated output and input about 3m apart, 2 signal lines. When using a straight, unshielded 3-wire cable (common return) in residential environment, I'm picking up an occasional spurious pulse, which triggers the input.

Replacing 3-wire cable with a shielded, 2 twisted pairs one removes the noise. Going down the cable quality ladder and using 2 pairs of Cat 5 cable also provides reliable transmission. Going even lower and using one line out of 3 different pairs (2 signal and common return) still provides a reliable transmission. At this point, I don't see what is the difference between using 3 random wires from Cat 5 cable, e.g. pins 2, 4 and 7, versus a straight unshielded 3-wire sensor cable. Could it be pair bundles twisting?

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  • \$\begingroup\$ By using one wire of each pair you also add a bit of capacitance to the signal wire which might ever so slightly clean up the noise a bit (slower edges and so on). So it might not necessarily be the twisting but just the extra copper around. \$\endgroup\$ Commented Nov 11, 2024 at 15:11

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Do Cat 5 cables have an additional twist between twisted pairs?

I have not seen it any specification saying they have to. But that's not to say that some won't do this. It's just probably has nothing to do with noise immunity.

The CAT-5 cable standard can be found in ANSI/TIA/EIA 568-B. You can look for yourself there.

Going even lower and using one line out of 3 different pairs (2 signal and common return) still provides a reliable transmission. At this point, I don't see what is the difference between using 3 random wires from Cat 5 cable, e.g. pins 2, 4 and 7, versus a straight unshielded 3-wire sensor cable.

Since there are 8 wires in a cat 5 cable and they are twisted, the average spacing between each of the wires (2,4,7) may be greater than what you were getting with the three-wire cable. If the wires are further apart the coupling would be less. It's possible that that's the difference.

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    \$\begingroup\$ Also I have not seen it in practise when stripping the jacket off cat5 cables, unless it's a very slight twist like 1/4m \$\endgroup\$ Commented Nov 11, 2024 at 3:04
  • \$\begingroup\$ Good point about wire spacing. \$\endgroup\$ Commented Nov 11, 2024 at 4:31

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