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What's DMX512


In the good old days of stage lighting control, we had large manually operated autovariacs to control the lanterns that lit up a show. These were placed next to the stage and some amounts of power cable snaked their way around the stage area to deliver actual electricity to the lamps. Not that the amount of cable was much but on the whole quite cumbersome; you can imagine ..... one needed an octopus's constitution to operate these things and forget about setting scenes !!

Until some smart guy fitted the autovariacs with motors ...and hey presto, you now had a smart panel back of house and the variacs were down in the basement ,motor and all! Only some low gauge control wiring to operate the motors went back to the console. The only hassle was speed...or slope, if that's what you would say now ! Motors had finite speeds and that was that!!

Then came the first Electronic dimmers and the first consoles. Necessity is the mother of inventions and the presets (or duplicate sets of faders for each channels) arrived.

Control was done using a small dc voltage, the proportion of which turned on the lamp to different levels of dimming. This voltage ran along individual wires for individual channels and this 'analog' system is still used all over the world. Different voltages and polarities are used but the +10 volt system is the most popular. This system suffers from two major problems:

1) It is prone to noise and earth loops if not wired properly over long distances.
2) It can be very non linear with the different kinds of lamps in use today.

Solutions to overcome these and other problems were attempted but were not remarkable.

Then came the basic computerised consoles using simple scene storage facilities. Outputs were still analog and improvments were done to send multiple signals over the same set of wires. Computers opened up a new dimension to the whole system ...... a fader need not be dedicated to a particular dimmer ; it could be assigned to any dimmer or set of dimmers. With the faders and buttons on one side and the dimmers on the other side, the computer could be made to compute any connection,level or slope that was required between them. The processors available at that time were slow so a lot of constraints were still there.

Different manufacturers came up with different consoles with improvisations of all kinds. They were quick to realise that a digital communication system between consoles and dimmers was a natural extension of the computer's power since it was spitting out digital numbers anyway. Various protocols were adapted with the result that there was little or no horizontal translation between manufacturers. It also meant that you had to buy all the gear from the same guy! The end user was the victim and a standard interface was very desirable under the circumstances.

The U.S.Institute of Theatre Technology (USITT) first developed the DMX512 protocol in 1986 as a standard interface between dimmers and consoles. It was a simple concept and was easily adoptable by all concerned. Since the first standard, some improvements was made in 1990 to accomodate some problems and, recently, further improved. It's now known as DMX512-A. It started out as a means to control dimmers from consoles and has ended up being used to control intelligent lights, LED lights, yokes, strobes, smoke machines, lasers and even confetti dispensers.


Although details of how USITT DMX512A (hence forth refered to as DMX) is described in detail on other dedicated pages , a preamble is given below.

The DMX protocol consists of a stream of data which is sent over a balanced cable system connected between the data transmitter (usually consoles) and a data receiver (could be dimmers or any of the stuff mentioned in the above paragraph).

A single DMX port, outputting this stream, can pass magnitude value information for a maximum 512 channels (or lesser) only. This port is known as a DMX universe. For consoles catering to more than 512 channels a second universe ( and therefore a second port) is required. The following is the universe/channel table:

1 1-512
2 513-1024
3 1025-1536
4 1537-2048
5 2049-2560
6 2561-3072

......and so on.

There is no restrictions on the number of universes.
Modern lighting console, for instance, have as many as 6 DMX512 universes and, hence, 6 DMX512 ports on their rear pane & can feed 512 x 6 = 3072 dimmers or dimming channels!! With the introduction of transporting DMX512 over LAN networks like ARTNET much more than 6 universes can now be sent.

I suppose processing speeds would be the only restricting factor in accomodating more DMX512 universes. We will consider the operation of the 1st universe only in these pages. The basic operation is the same for all universes except for the physical channel numbers at the console end. It is important to know that receiving devices will have circuitry to recognise only channels 1 to 512. In multi universe systems, the LD and riggers have to physically identify universe/channel combinations when wiring up receiving devices. For example, a dimmer which needs to be connected to the output channel 2331 of the AVO DIAMOND II, would be connected from the 5th universe output but would have an address of 283 set on it. That's because Output channel 2331 is actually channel 283 in the 5th universe.
The data stream is sent as a packet ( hence forth refered to as the DMX packet) of data which is repeated continuously. It consists of starting bits of data which informs the receivers that the packet is being refreshed and then sends out a stream of serial data corresponding to the magnitude value of each channel , starting with channel 1 and ending with 512 or a lesser channel No( depending on the design and size of the console). Each channel is separated from the other by specified bits of start and stop data.

The whole system works like a city postal system. Each postman ( universe ) has a beat of 512 houses(channels). Each house(channel) has a unique address.Some houses are high risers with many individual apartments(several channels in one unit like an intelligent light). The postman goes from house to house and delivers the mail(the value data ) in individual letter boxes. Each occupant opens only HIS letterbox and takes HIS mail. Similarly each receiving unit is told it's address (one of 512 adresses) and thus ignores all other data except the one it's supposed to receive against it's address.Some units like intelligent lights have one address as a start and go on to receive data for that address AND several more addresses FOLLOWING it . Not unlike the lobby security receptionist who takes in the mail for every one in that building and then distributes it.

The data stream has a specific format
( DMX512 PACKET) and has specific physical properties (DMX512 ELECTRICS) by which it is propagated .

&h01... &h02... &h03... &h04... &h05...

&h06... &h07... &h08... &h09... &h0A...


Copyright: Ujjal Kar | ujjal@standardrobotics.com

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