What's Up With XLR Cables?

umm ok,
i just use normal patch cables. i only know there to be one kind of 3 pronged patch cable. so i am assuming they are all the same.
i have noticed that on the DJ Scans there is no lock pin on the female socket (OUT) like on most other fixtures or appliances that use XLR patch cables.

Okay, a couple of issues here.

First is the Shield/Connector Shell Issue. The reason for that is that you don't want the shield, which is you SIGNAL GROUND to touch you CHASSIS GROUND, which is the ground for the fixture. If you get a Chassis ground Signal ground touch you can send some serious current up the shield and a lot of the lower cost equipment can't deal with that. Any DMX cable wont have and connection to the shell, some Mic cables do, but you shouldn't be using Mic cable for DMX cable.

Second issue is the cable. DMX cable is totally different from Mic cable. For DMX, you should use a cable that is certified for EIA-RS485 applications. Some examples are-Belden 9841, TMB Proplex, Creative Stage Lighting's Duraflex. DMX cable wants to have a caracteristic impedence of 120 ohms and has a very small capacitance per foot. DMX is a digital signal, anything under 2.5volts between pins 2 and 3 is a zero, and anything over 2.5 volts is a one. When you use Mic cable, which has a much higher capacitance per foot, the voltages get averaged and you can have some problems.

The last issue is Termination. DMX is transmitted at 250 kbs. That is technically a radio frequency, and when it reaches the end of the data run, you can get a reflection that travels backwards down the line and can confuse the receivers in the fixtures. The terminating resistor stops this reflection. Just solder a resistor that maches the caracteristic impedance of the cable. If you use actual DMX cable, it would be a 120 ohm resistor between pins 2 and 3 of a male 3 pin XLR plug. You can get them pre-made.

I hope this helps explain stuff. If you would like info on where to buy DMX cable just let me know.

Paul DeRocher
Technical Service Manager
ALPS/Advanced Lighting
paul_derocher@alpsweb.com

True DMX cable is made out of Cat. 5 like cable. But sometime expensive and hard to find made up. But this cable is mostly used for higher end fixtures. Good Xlr cables will do the job.