For controlling the field equipment, I need to circulate 24vDC from my RTU to the field equipment, and 110VAC from the other direction - For input and output.
In this case, can I use a single 2pair cable with one pair carrying AC signal and 1pair carrying DC signal. Note that the current is going to be in a very few mA (typically contributing to the resistance in the circuit) and the cable distances are not going to be more than 100meters
Thanks for the answer.
But it is still not clear, what would be the effect of 110VAC on the 24VDC?
If the pairs were not twisted, the AC voltage would be capacitively coupled to the DC pair, and the AC current would induce in the DC pair an AC voltage as well, caused by the changing magnetic flux. The idea of twisted pairs is to change the direction of the effect, depending on the orientation of the pair, resulting in a mean effect of nearly zero.
I never tried your setup, but I can imagine one bad case: If the two pairs have nearly identical geometry it might be that the "twisting" of the AC field and the twisting of the other pair result in a stronger coupling than in the general case. So, I feel that a cable containing one twisted and one not twisted pair would be better.
However, if you use the DC voltage exclusively for power supply it should be easy to block a possible AC component at the receiver end.
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