If you're using intelligent lights, dimmers or other things that you need to directly control, chances are you're using DMX-512 and some sort of DMX controller, be it hardware or software. There are a few things you need to stop and consider:
1: Please, and this is VERY important, you need to be using DMX cabling. While 3-wire microphone cable CAN work, it is rated for 1000-600 ohms and is designed for microphones. DMX cabling is 120-110 ohms and is designed for data applications. 3-wire DMX is wired identical to XLR microphone cabling, which only adds to the confusion. Just because it CAN work doesn't mean you SHOULD be doing it. Please use DMX cabling. The cost differential is small at best, not enough for you to make the wrong decision.
For years I ran my lighting via XLR cabling. Yes, it worked. I'm done with that, I'm only using AccuCable for my lighting DMX signal now. Proof that just because you did something for years doesn't mean you were doing it right.
On the phone call, the guy said he spec'ed AccuCable, but the electrical contractor installed XLR cabling instead, saying it would be good enough. Hmm, if that was me, I would have fired the electrical guy on the spot. Unfortunately, it sounds to me like things got out of control without the caller being directly informed of such decisions. Now the sheetrock is up and making the changes will be difficult. Thank goodness they ran conduit, there's still hope!
2: Use a DMX Terminator. These can be bought or made cheaply. Chances are you only need one, but if you're mobile, make a handful because they can get lost easily. I know, I've lost my fair share of them, but I also make my own, which doens't help any. While ADJ and Elation state any runs over 100-feet of total cable length should for sure be terminated, I go ahead and terminate all runs regardless of length. Better safe than sorry. Actually, sorry isn't the right term. I think the worse that could happen is your signal reflects back into the wiring and you get some wonky behavior, but probably no permanent damage.
3: Protect yourself. Protection can be in the form of an Elation Opto Branch-4 or DMX Branch-4. These devices do many useful things. They can accept either 3 or 5 wire DMX, and output either 3 or 5 wire DMX per "Branch" and there are 4 branches pre box. This also means they can repeat the DMX signal across multiple ports. Also, since they do some sort of optical isolation, electrical badness that may happen at stage won't come back and fry your DMX controller or computer interface, which is very important(but rather unlikely but it has happened). Not exactly cheap, but they are still affortable. Check them out. I own the Opto Branch-4 and I use it with MyDMX. Great combination.
Another good application with my Opto Branch 4, I have one run that goes to the stage, another run that goes to my far tree and floor mounted lights, another run that goes to my near tree, and a 4th run that goes to the back to my follow spots. This reduces a few cables and reduces some clutter. This sort of concept is more for what these units were designed for. But more runs means higher chances of something coming back at you that you don't want. Again, the Opto Branch-4 and DMX-Branch/4 address all those concerns.
4: In regards to MyDMX, it doens't put out the strongest of DMX signals, so using a DMX Branch/4 or an Opto Branch-4 ensures you have proper signal strength for DMX. Again yet another reason to buy one of those units. But, this can be said for other hardware interfaces for entry level products.
5: Interconnectivity. I just mentioned there is 5 and 3 wire DMX. The only thing you need is an adaptor cablem which you can make yourself, should you need it. There is no electrical magic, it's just a 3-wire circuit. Or, units like the DMX Branch/4 and Opto Branch-4 can do this for you. This isn't always practical to have your repeater handle this based on how you're running stuff. That's where you need adaptor cables. If you only run one type of DMX wiring, then you don't need this advise. But if you do make adaptor cables, make sure you make ones in the reverse pin terminations as well so you can adapt your way back out.
6: Polarity: Some lights are wired backwards. Seems odd, especially when the DMX specs state how things are supposed to be wired. This is an issue that can drive newbies insane with their new lighting. It's a tough one to diagnose. Some controllers have a polarity reverse switch, which can help troubleshoot or get the backwards-wired lights working, but then your properly wired stuff will stop working. The solutions are to make and LABEL and DMX cross-over cable(reversing 2 and 3 positions on one end). Make them in pairs so you can revert back to proper polarity. You have other choices. My choice would be to crack open the light and wire the connector properly. One clever uses chose to modify their Opto Branch-4 on one channel to be polarity reversed. While there is more than one way to address the problem, just be warned. LABEL your polarity reversing cables if you go that route.
I am fortunate that I do not have any polarity issues. However, sticking with ONE BRAND does not ensure you won't run into this problem.
Everyone's rig is different. Know your gear, and know you're wired properly and you're ahead of the game. Now go out and have some fun.