DMX cable Ohm????

[Datte83]

What is the correct ohm on a DMX cable?

Read on the web that some say 110 and other 120 ohm. What shall i choos?

 

[microstar]

It doesn't matter. Both fall within the specifications.

 

[Bubby4j]

But it's best not to mix both types, if you choose 110, all the cable should be 110, and if you choose 120, all should be 120. Will need a matching DMX terminator with the same resistance resistor.

 

[microstar]

But it's best not to mix both types, if you choose 110, all the cable should be 110, and if you choose 120, all should be 120. Will need a matching DMX terminator with the same resistance resistor.

So what do you do when you have piles of mixed up DMX cables? In a perfect world, Yes, in everyday use, No.

 

[Datte83]

thanks for your awnsers. Then I will go for the 120 ohm. ('Cause that's what the cable is that Im looking at to buy)

 

[Bubby4j]

So what do you do when you have piles of mixed up DMX cables? In a perfect world, Yes, in everyday use, No.

True, but it's still something to keep in mind.

 

[Datte83]

Do I need a DMX-terminator on the dimmer if I only use DMX for dimmer control ( lightdesk- DMX- dimmer)????

 

[Lsly]

Do I need a DMX-terminator on the dimmer if I only use DMX for dimmer control ( lightdesk- DMX- dimmer)????

Its never a bad idea. And can save your show if there is an issue. Many rack have them installed already, usually a switch right next to the 5-pin port.

LS

 

[JD]

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.

 

[MikeJ]

For how cheap a hand full of resistors and male XLR connectors are, you might as well terminate all of your lines. Just don't pushed the terminate button on a rack, and use a plugin terminator at the same time. That will result in a 60ohm termination.