Understand DMX Lighting and Fixtures

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Are you interested in DMX, or ever wondered how those cool lights at concerts worked? This article explains the working of these fixtures and the protocol.

Steps

  1. Find out as much as possible about the protocol first. DMX stands for Digital Multiplex interface. Each universe 512 channels and is wired using 3-pins or 5-pins. A great web site is Ujjal's DMX-512 Website which tells you everything about DMX that you need to know.
  2. Learn how to address channels (see Use DMX Addressing)
  3. Understand the fixtures. You may need this info if you decide to build a fixture of your own.
  4. Know how to overcome problems that may come up in the rigging, programming, rehearsals, and showtime.
  5. Lastly, but very importantly, know how to work/program the controller. Controllers range from 6-channel lightboards to 6 Universe lightboards, and cheap computer DMX dongles to Whole Hog systems.

Tips

  • Make sure to test your equipment BEFORE any type of rehearsal or show.
  • Don't forget to terminate all you lines or else you will get erratic signals (eek!).
  • There are many types of DMX fixtures, but just to name a few
    • Dimmers: These control par cans, fresnels, Source 4, etc. They vary the voltage to dim the fixture.
    • Gel Scrollers: These mount to the front of par cans, Source 4s, etc. and scroll the gel (color sheet) to alter the color
    • Scanners: These are actually intelligent fixtures. Scanners do not move like moving head/yoke, but instead have a moving mirror. Sometimes called moving mirror lumineres They usually have the ability to: 1. mirror pan; 2. mirror tilt; 3. color wheel; 4.gobo; 5. strobe; And sometimes: 6. pan/tilt speed; 7. frost/effect; 8. dimming; 9. focus; 10. iris.
    • Moving light/ Moving head/ Moving yoke: Most common of fixtures, yet also most expensive as up to $25,000. They usually have the ability to: 1. pan; 2. tilt; 3. pan fine; 4. tilt fine; 5. gobo; 6. gobo rotation; 7. frost; 8. dimming; 9. focus; 10. iris; 11. strobing; 12. prism; 13. prism rotation; 14. effect one; 15. effect two; 16. framing. plus lots more of channels
  • It is prohibited to use DMX to control any life threatening objects such as pyro, truss motion control, etc.
  • Never use mic cable instead of 3-pin. It WILL mess your show up at the wrong time.
  • DMX operates with a system of packets. The packets are:
    • IDLE or NO DMX situation: In the absence of a valid DMX packet the output of a DMX line will be a continuously HI signal.
    • BREAK: The start of a DMX packet is heralded by the output going LO for a MINIMUM period of 88 microsecs. This means 22 LO bits are measured out one after the other. This is known as the BREAK. The BREAK could be longer but not less than 88 microsecs. Experience shows that slightly longer breaks (above 88 microsecs) sent from a console are better since the receiving devices are generally given the algorithm = "Is the BREAK>88 microsecs or 22 pulses". I keep it generally at 100 -120 microsecs in equipment designed by me
    • MARK AFTER BREAK (MAB): The MAB immediately follows the BREAK by making the output go HI for a MINIMUM period of 8 microsecs or 2 pulses. This MAB is a bit of a problem since the difference between the original DMX512 and the current DMX512(1990) standards relate to this period of the packet. The original was set at 4 microsecs or 1 pulse. This created hassles for some receivers for being too short a MAB for detection and was upgraded to 8 microsecs or 2 pulses in 1990. The problem comes when an older console is used with newer receivers or vice versa.Wrong detection will lead to packet rejection or the wrong data going to the wrong channel. This will travel down the line leading to utter confusion. Some receivers have a dip switch to set this parameter for both timings. Again the maximum MAB length could be 1 sec. One ideal timing would be 12 microsecs.
    • START CODE (SC): The SC is next in the line. It is easier to remember that the SC is the start of the actual data stream where all individual channel data have the same format. The BREAK & the MAB were of different timings but the SC onwards all frames will have the same structure and timing of 11 pulses or 44 microsecs width. The first one can be termed as data for channel No 0 which is a non-existent channel and represents the SC.

Warnings

  • Be careful when rigging, most fixtures weigh a lot.
  • Sometimes DMX lines can shock you, so watch out for an occasional tingle.

Sources and Citations

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