The second twisted pair does absolutely nothing for data transmission. If the standard is okay to use
5 wire dual twisted pair and omit a pair.. that is the same as using 3
wire cable with a twisted pair of the same spec
wire. Or at least that is my believe from the several talks about
ETC's standards
What Gern is talking about is not the number of pins, but the quality of the wiring. The average analog
microphone cable has an
impedance of roughly 40-50 Ohms and has a
capacitance of 30-120 pF/ft.
Per spec,
DMX cable is ideally 120
Ohm characteristic
impedance data-grade cable with a
capacitance of 12.8pF/ft. The number of pins is irrelevant.
There are more technical details to a cable than that the wires are made out of copper and the number of wires a given cable has. The average
microphone cable is non-ideal for
DMX because it it is of the wrong characteristic
impedance and
capacitance.
That said --
DMX tends to be very forgiving. As noted in
Doug Fleenor'
s video, you can run
DMX over three strands of barbed
wire (it's actually been done), and it'll work really well until it doesn't work at all. When it stops working, first you'll blame your lighting
console, then you'll blame your fixtures, and then you'll spend a few hours swapping around your cables to find "the bad one", but if they're all audio-quality cable, they'll
all be bad and corruptive to your
DMX signal quality, and cause your fixtures to be subject to spontaneous and erratic behavior.
Audio-quality cable can work fine for
DMX for an extended amount of time, and then the slighest variable changes and no matter how you order your cables in the
daisy chain, your signal gets corrupted. Could be a walkie-talkie that sets it off, or could be one too many fixtures are on the chain. Whatever the problem, you'll lose more than a couple hours trying to
chase it down.
Thus we do things like limit the number of fixtures in a chain, use data-quality cable, and use 120
Ohm terminators, and never split a signal via a wye-splitter. You can probably get away with all of those kinds of sins, but you're living on the
edge of never knowing when your signal may suddenly cause all of your devices to take on a mind of their own mid-performance.
Belden has a good document for selecting the appropriate cables for
DMX, and here are some technical data sheets so you can compare the kinds of details that are at
play from one type of cabling to another:
DMX Single-Pair
DMX Dual-Pair
One Kind of Microphone Cable
Another Kind of Microphone Cable
