LIGHTING EQUIPMENT
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DMX 512 EXPLAINED:

DMX 512 Equipment

 

The original intention of using DMX512 for controlling dimmers only, has now been stretched to include a whole range of equipment. The 8-bit data structure, which was originally used to specify 256 levels of dimming only, is now also used to define many different parameters in different equipment:

 

- Mirror position
- Gobo position
- Gel string position
- Shutter position
- Focus motor position
- Smoke machine pump pressure
- and many more.

 

Similarly, equipment for generating DMX512 have also taken different forms, although, the original basic console is still very much there:

 

- Special console functions to facilitate use of moving lights
- Personal computers (IBM or MAC)
- Stand-alone equipment for Architectural lighting
- Back-up equipment for consoles
- Test instruments
- and many more.

 

Some equipment both receive AND generate DMX512:

- DMX protocol converters
- DMX multiplexors and de-multiplexors
- DMX splitters
- and a few more

 

We'll do a quick run through some of them:

 

CONSOLES:

The console converts data collected by it from the various tactile controls on it into a 8 bit form using Analogue to Digital converters or other devices and then computes the required output data. This computation may be simple or complex depending upon the desk functions and it's patching scheme. The final CHANNEL values are inserted into a DMX512 engine after the processor correlates with the patch table inside it. The DMX512 engine outputs the values after sorting them into respective DMX512 Universes.

 

DIMMERS:

Dimmers receive DMX512 data and use it to control the width of a pulse which then triggers a power control device such as a triac or a set of thyristers. The pulse is synchronised to the AC mains in a manner that allows the power device to control the phase of an electrical current passing through it, thus varying the power supplied to any lights that may be connected to it in series. However, the actual light output-to-phase angle ratio is not the same for all lamps in practice and, therefore, the DMX data-to-phase angle ratio is often compensated over the entire 256 step (8 bit digital) range. These compensating curves are called dimmer curves and are often user selectable in the dimmer or from the console.

 

MOVING LIGHTS:

Moving lights, scrollers, yokes etc., work on more or less the same principal and are being discussed together. The central idea in all of them is to use DMX512 data to determine the relative angle of a motor shaft. This is achieved in a number of ways...the most common is to use a stepper motor (similar to the fella who turns your computer printer roller to feed the paper or makes the print head go left to right & back, while printing) The stepper motor is an electric motor which turns it's shaft a certain little bit when you put a momentary pulse of electricity in a certain set of wires and in a certain order. Reversing the order of pulsing makes the shaft turn in the opposite direction.

 

There may be several such motors turning the mirror, gobos, lenses, lamp shutters etc in a moving light. Each needs its own channel of DMX data and thus separate channels are used. The data is received from one channel address onwards until reaching the number of channels required. It is then separated and passed on as the required number of pulses in the required order to the motor concerned. The Scroller has two such motors with a reel shaft on each, across which the Gel string is strung. Based on the DMX512 data received, they reel the Gel string in or out, changing the colour coming in front of the lamp. Similarly, other equipment uses different mechanisms using these motors to achieve the necessary end.

 

Other than stepper motors, servo motors are also used where a closed loop feedback on the shaft position is obtained. In addition, a channel may be used for a traditional dimmer (as explained in DIMMERS above) inside the Moving Light to vary the intensity of its lamp. However, this dimming is also done mechanically, using one or two stepper motors driving a pair of shutters across the light path.

 

OTHER STUFF:

DMX512 is also used to control smoke machines. One or more channels are used. The basic channel is used to trigger the smoke machine to blast away by allocating an 8 bit value OVER which the pump inside activates. One more channel may be used to regulate pump pressure when its turned on by the base channel thus controlling smoke quantity.

 

By using DMX512 to activate relays, you can make it control Confetti dispensers, Snow machines, Strobes and many other such appliances!!

 

WARNING:

The following equipment are SOME of many which are prohibited from using DMX512 as a trigger source as specified by the standard:
1) Pyro stuff
2) Set shifting stuff
3) Truss motion control
In fact, any thing which compromises the safety of human (or animal!) lives due to failure to receive and interpret DMX512 correctly, is prohibited.

 

When billions of dollars have been spent on technology research to get the desktop PC to the common man, why waste money & time re-inventing the wheel?!

The power of the PC can be used to generate spectacular end results as far as DMX512 is concerned. What you need is an outboard USB / E Parallel Port or add-on card to generate the DMX512 signal and software to power it up. The 250khz DMX clock can be obtained from the PC's clock line. The main computation can be controlled by the PC, and the DMX portion can be handled by a separate dedicated microprocessor which continually outputs DMX and gets a refreshed DMX page from the main CPU at intervals. With a DMX512 Input facility, a dedicated tactile interface or a fully fledged console can replace runtime operation with the otherwise cumbersome PC keyboard / Mouse.

 

 

-DMX PROTOCOL CONVERTERS are used to convert other protocols (AVAB, PMX, D54, MICROPLEX, etc) to or from DMX512.

 

-DMX ANALOG MUTIPLEXERS convert DMX512 data to and from analogue systems.

 

This may be multiple analogue (+10v / -10v / +5v) or multiplexed analogue (e.g. D54) signals.

 

-DMX SPLITTERS generally provide multiple DMX outputs from one input and can drive a large number of units. Some splitters provide the same input data on all out puts OR divert the input data in a split fashion to two or more outputs.

e.g. channels 1 to 256 may be on the first output and 257 to 512 on the second one. Thus channel 257 becomes channel 1 on the second output.

 

DMX MERGE units take two or more DMX512 inputs and merge them into one DMX output stream placing one input stream AFTER the other.

e.g. If two DMX inputs ( say A & B) are merged then the output (C) will first send A then follow it with B using only one BREAK / MAB / SC sequence in the beginning.

Thus if A+B = C then C (1 - 256) = A (1 - 256) and C (257 - 512) = B (1 - 256)

 

DMX MIX units take two or more DMX512 inputs and mix them together channel to channel and produce one DMX512 output. The mixing is usually done in the HTP (Highest Takes Precedence) mode. Which means whichever of the two inputs are highest (in terms of value) is passed on to the output.

 

Thus C (n)= A(n) OR B(n) ,whichever is higher.

 

Note: If A has 512 channels and B has 200 channels then C will have 512 channels of which the first 200 will be mixed. Channels 201 to 512 of A will pass on to C , unaffected.

 

 

This generally takes the form of small hand held boxes with DMX IN and DMX OUT connectors. They can address any of the 512 channels separately or collectively with selectable data and are fitted with suitable LED or LCD screens to display details. They can also receive data from any selected channel and display the same. They may have other advanced features like measurement of BREAK, MAB, SC timing or PACKET refresh rates and built in diagnostic warning signals in case these parameters deviate from normal range. They also have trigger outputs (+ or -) for oscilloscopes which is useful in order to view the correct required frame while servicing DMX512 equipment. It's the 1st thing you should get if you are going to use DMX512 stuff.