DMX. That now ubiquitous way of controlling our lighting fixtures. While the standard of DMX is pretty well established—at least in terms of the speeds, number of channels and controls—there are still plenty of variables. Fixtures can have 3-pin XLRs on them, or 5-pin. Or both. Sometimes, the in and out jacks on a fixture are simply wired together and it doesn't matter which is which. Other times, they are active and you can't plug in to the out jack and visa versa. And while you can get away with running DMX over mic cords for shorter distances, it's not recommended for longer runs.
Over the next two posts, we're going to talk about a few ways to run DMX from FOH to fixtures. In this post, we'll look at using Cat5 cable instead of DMX cable. Next time, we'll consider using a DMX networking system, which also used Cat5, though in a very different manner.
Now, I should point out that when we're talking about using Cat5 for DMX cable, I am referring to the longer, backbone type runs in a system, not fixture-to-fixture jumpers. In most DMX systems, you will have a run or two (depending on universe count) from FOH to the stage, then the signal hits an Opto-splitter and is distributed throughout the stage and house lighting. At least that is what should happen; I have seen systems where there is one run from the console to the first fixture, and the DMX is daisy chained throughout every single system in the room.
While that can work, it's not ideal. Once you get past about 18 fixtures in a DMX chain, things can get weird. Not always, but sometimes. For this reason, it's better to keep the fixture count lower and use a proper splitter to give you multiple branches of the DMX universe in your room.
I should also point out that a terminal block or wire nuts do not a proper DMX splitter make. You really want an active signal splitter that not only sends out an exact copy of your DMX signal to each port, but also isolates the ports from each other. That way if you have a problem on one branch, the whole system doesn't go down.
With those disclaimers and background, let's consider the first way we can use Cat5 cable in our DMX system.
The most straight-forward use of Cat5 in a DMX system is just a simple cable replacement. Instead of pulling standard DMX cable through the conduit, pull Cat5. There are several advantages to this. First, Cat5 is a lot cheaper than DMX cable. Second, it's a lot easier to pull than most DMX cable due to it's slippery outer jacket (one that's designed to be pulled through conduit). Cat5 is also readily available in long lengths just about anywhere.
When using Cat5 cable in a DMX system—really any cable—it's important to follow proper termination procedures. You can solder Cat5 cable to XLR plugs, but it's important to pay close attention to the cable pairs and pinning. The folks at Pathway Connectivity provided this chart, which I've used for all my jobs with great success.
So let's look at a simple system as an example. We would come from our lighting console to an opto-splitter, then out to each branch of fixtures over the stage or house. This is a simple, single universe system, so we'll pull one run of Cat5 from FOH to the stage where the opto-splitter lives, then run standard DMX cables to the fixtures.
As you can see, it's pretty simple. Next time around, we'll use Cat5 to it's full advantage, and I'll show you how you can get 64 universes on a single Cat5.
By the way, I built this whole post on my iPad while stuck in an ice storm in Nashville.