Many faults in DMX systems are caused by simple cable or fixture addressing problems. With a little forward planning, you can have most small systems working straight out of the box by knowing how to address your fixtures and having the correct cables on hand.

Let's start with the cable. DMX signals should be carried down screened twisted pair (STP) cables in an ideal situation. Having twisted pairs is the important bit here, as this helps to ensure that noise is picked up equally by both cores of the cable and then can be rejected by the receiver within the fixture.

For longer cable runs, it is important for the cable to have a low capacitance, what I'm really getting at here is that you shouldn't be using microphone cables for long DMX runs. In certain situations it will (and has been known to) work without problems, but the results can be unpredictable, and what may work on one occasion may well fail on another. Remember that microphone cable is designed to run at audio frequencies probably not reaching far over 25Khz, not at the 250Khz frequency used by DMX!

Again, good screening will help on long cable runs, although tests by ESTA have shown that unshielded twisted pair (UTP) cable, as used in computer networks, will give perfectly acceptable results in many situations. This cable is now very common (and cheap), and would be ideal for anyone making DMX leads on a budget. Remember to make sure that you get cable with stranded cores, unless the cable is for a permanent installation. If you can get foil shielded STP cable, even better!

Another important issue is the earthing of the XLR connectors on your DMX leads. When fixtures are placed some distance apart, or in a poorly wired system, there could be a large difference in potential between the earth connections for each fixture. If you happen to connect the earthed metal casings of two fixtures with your DMX lead, and there is a difference in earth potential between the two, a current will flow down your DMX cable. In extreme situations where there is a very large difference in earth potential, dangerous currents could flow that are much larger than the cable was designed for. This could easily cause the cable to heat up, melt, or even catch fire!

Fortunately this is easily avoided. You just need to remember that the only place that your DMX lead should be bonded to earth is at the console. Nowhere else. When wiring your leads, connect the cable shield to pin 1, and wire the two inner cores in the usual fashion. Don't wire the XLR body to earth. As long as the fixture you are using does not internally connect pin 1 to the fixture body earth, everything should be safe.

Another common problem is that some manufacturers reverse XLR pins 2 and 3 on their equipment. This will cause the fixture to detect invalid data, so spend a little time and make some leads that will reverse the polarity of the DMX line and save yourself a lot of trouble in the future. Such a lead would be wired as shown below:

XLR1    XLR2
Pin 1 > Pin 1
Pin 2 > Pin 3
Pin 3 > Pin 2

That's most of the issues covered as far as leads are concerned. Also remember to put a termination plug (or flick the termination switch) on the last fixture in the chain. You can make a termination plug yourself by soldering a 120 Ohm resistor between pins 2 and 3 of a male XLR. Sometimes a system may work fine without this termination resistor, sometimes it wont, better to be on the safe side.

As for addressing, just make sure you know your binary for setting those DIP switches! Remember that a 5 channel fixture set to address 7 will occupy channels 7-11, so the next fixture in the chain should start at channel 12. That is, unless you want both fixtures to copy each other, in which case you can happily set them both to the same channels.

Hopefully this article has cleared up a few issues. If not, thanks for reading anyway! It's getting a little late now, so if I've managed to make any stupid mistakes please drop me an E-mail and I'll sort them out ASAP.