Mixing 120
ohm and 110
ohm cable should not be a problem. Remember, these are "transmission
line"
impedance ratings that are only fractionally relevant at the lower frequencies that
DMX operates at. However, don't mix them with mic cables or other wildly different
impedance cables or you may have some surprises! Typically, digital cable like
Cat5 is 110
ohm. Most of the commercial
DMX cable is 120
ohm. These characteristics reflect that the cable in transmitting a high frequency signal actually responds like a series of chokes and capacitors wired in series. Any "reactive"
line has a natural
impedance. The higher the frequency, the more critical this becomes. (Measured with a
ohm meter at DC, the value does not exist.) So,
Cat5 is rated to 1Ghz.
DMX is about 245Khz, or 400 times lower in frequency, and that much less likely to be a problem.
In a perfect world we want to remove all variables, so you would have everything the same. In the real world, the natural manufacturing tolerances of the cable would be greater than the variance between 110 and 120
ohm cable. The general construction of the cable becomes a bigger factor. Mixing in 60
ohm or 600
ohm cable would open the door to potential problems. Using good connectors, proper terminators, and
opto splitters to reduce
ground loop and isolate failure paths is far more important.