DMX

[Casual5]

Greetings Fellow Blue Room Forum Users,

 

My post is about DMX.

 

I am a graduate of Sound and my knowledge of lighting and other controlled visual effects in the theatre world isnt the greatest.

 

If any of you guys can spare a couple of sec's could you explain a wee bit more about the world of DMX for me, in particular I would like to know what a buffer is and what a terminator is used for....

 

I know there is probably information about this in books and on websites - but I am quite keen in the participation of these forums and obviously so are you.

 

Ta.

[SceneMaster]

A terminator is simply a plug with a 120ohm resistor between pins 2 and 3 which is used at the end of a DMX line to stop what is effectively “reflections” of the data being reflected back down the data lines.

[Ike]

As said there are hundreds of resources on the net relating to DMX 512. DMX is a digital multiplex protocol which uses the EIA485 (RS485) electrical standard and should be used with screened data cable consisting of one or two twisted pairs, microphone cable should not be used due to it's incorrect capacitance. The correct connector is the five pin XLR wired pin 1 shield, pin 2 data-, pin three data+, pin four spare data- and pin 5 spare data+ however three pin XLRs are now seen as acceptable in some circles. Martins pre Mac moving lights and a number of others from cheaper manufacturers reversed pins two and three in their three pin XLRs however this problem is now thankfully getting rarer. DMX networks should be set up as a daisy chain with a maximum of 32 receivers, larger setups may use multiple chains connected to a splitter. Terminators are used to prevent data reflections which are most often seen as jitters in scrollers and moving lights, the desk usually forms one terminator with the other consisting of a 120 ohm resistor across pins two and three at the other end of the chain.

 

DMX is transmitted at 250K baud or 250,000 bits per second and gives values from 0 to 255 for 512 channels. If more channels are needed more than one DMX "universe" must be used and if more accurate control is needed (for example pan and tilt on some movers) two channels are used for those particular attributes.

 

If you wish to convert from DMX to +/-10V analog control then you will need a demultiplexer or demux.

 

DMX fixtures and dimmers must be set up to respond to the desired addresses, this is often done via dip switches or a simple push button and LCD interface.

 

Have a look at here for instructions on how to make a basic (but very useful) DMX tester for a couple of quid!

 

Edit:Spelling

[Ekij]

Casual 5, read the Wiki entry

Blue Room Wiki for DMX

And come back if you have any further questions.