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Subject: [LCtalk] DMX nightmares; fast DMX vs slow DMX (long)
From: Fokko van Duin
Date: Tue, 05 Oct 2004 12:10:10 +0200

Hello all,

DMX-512 is -the- standard protocol in the lighting world, but we all know it can be a very frustration experience when you face 3 and 5 pole XLR connectors and pin 2&3 swap. Still, if you use good cables, have a bag of 3/5 pole and swap adapters, it's working perfectly most of the time.

As long as all devices (controller and fixtures/dimmers) conforms to the DMX-512 USITT 1990 standard there are no further problems, but nowadays we experience more and more incompatibilities.

What is going on?

The DMX standard allows controllers to create a signal which is a very fast (44 times/sec) repeating packet of 513 bytes. This gives the lowest delay between channel 1 and channel 512, and also the fastest response when changing DMX values. As this "fast DMX" is very difficult to create in software, most desks and controllers reduce the packet rate to 20 packets/sec, which still gives good response times. (The LCX has 23ms delay between ch1 and 512, and a response time of 50mS)

DMX receivers (fixtures, dimmers) -must-be able to handle the highest DMX speed without problems, this not only sounds logically, but it is demanded by the standard as well.

However...

As intelligent fixtures becomes a mass product, some manufacturers simply reduced the speed of DMX in order to save some money in the design. They discovered that their fixtures are still working on most desks, while not conforming with the standard anymore. This is possible as most desks increases the waiting time between the bytes (higher delay) when they send less then 512 channels. Also the same manufacturer of the fixtures also makes desks, which are naturally made so their fixtures works.

This types of fixtures are -NOT- DMX-512 USITT 1990 compatible, and there is a good change they will not work correctly with a LanBox or a decent 512 channel output desk! So be sure you buy/rent fixtures which have a written statement that they fully comply to DMX-512 USITT 1990.

What can we do about it?

First of all, you as customer of fixtures should NOT accept non-standard DMX fixtures, but the fact is that there are coming more and more "el cheapo" fixtures on the market. Several manufacturers have even declared "off the record" they don't care the standard, as long as it works on the majority of desks.

So, we are forced to make "slow DMX" in our controllers?

The Lanbox controllers where designed to output fast DMX, in order to have the best performance. Lately we have degraded the LanBox-LCM and LCE to a bit slower DMX, so that most of the non-conforming fixtures/dimmers could also work. This is still not slow enough for some fixtures, and the LanBox-LCX can not easily be degraded to do this.

We are currently investigating if we can make our controllers with a slow DMX option, but you will never be sure that this will always work with these low cost (slow) fixtures.

We are very interested to hear what you think about this whole mess, and what we should do about it.

Please respond, as one day you will also be hit by this phenomena :-)


Fokko van Duin,                     Creators of the LanBox-LC
CDS advanced technology bv,         DMX lighting controller.
http://www.cds.nl/                  http://www.lanbox.com/

Member of MIDI Manufacturers Association, MMA
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