DMX for Dummies?

[LXRob]

Hey Im sure there must of been threads on this before, but it won't let me search properly. So if anyone can explain DMX from the ground up tat would be ace! or redirect me to somewhere I can find this?

[Wol]

Just look on the handy BR wiki using the direct link. No need to search for specific threads.

http://www.blue-room.org.uk/wiki/DMX

 

Or go and have a google search. A good site is http://www.dmx512-online.com/packt.html

 

Was there anything in particular you wanted to know?

[pritch]

I've got to second what Wol says - 'ground up' could be a really big question, depending on where you consider the ground level to be! Do you need to know the exact details of how the protocol works, down at the level of individual bits? Or are you more interested in how you can string a bunch of fixtures together and get a desk to talk to them?

[paulears]

Blimey!

 

There are tons of topics on here about DMX - but if you want the ultra basic explanation, I'll try. The basic idea is that a lighting control system has an output that can carry the data containing the information the fixture or dimmer, or other device needs. All these separate messages (512 as a maximum) travel down the cable, and each device is set to only responds to one (in the case of a dimmer) or a cluster of them (in the case of a moving head complex fixture). The cabling is fed to each unit in turn, in a kind of daisy chain. Each one having and in, and an out. So the lighting desk sends out instruction to every unit down a single cable, and each device 'listens' out only for ones that it should respond to. Dimmer channel 1 only responds to messages on channel one - the other 511 just get ignored. A simple moving mirror scanner might need 4 channels - left/right, up/down, colour and gobo. So if you patched this to channel one, then the next free address for the next fixture would be DMX ch 5, where you could patch the next one.

 

If you only have dimmers, 512 is a lot. If you have a matric type LED bar, then that one fixture might use up 24 channels of the 512. This means in practice, 512 may well be not enough - so they provide on more complex desks multiple outputs, often referred to as 'universes'. so a control could have 1,2,3,4 or dozens of output universes.

 

DMX isn't interesting or clever - it just a connection system that has become a standard (almost) and pretty well anything can be patched. Problems are well documented. The end of the daisy chain should be terminated with either a switch in the fixture or a terminator plugged onto the last empty socket in the chain. Most times, doing without this doesn't cause problems, but the system can be a bit unstable - and you can imagine the results if a message on ch4 gets mistaken for 44 - weird flashes, flickers and strange events. terminating the line usually keeps things stable. The DMX spec calls for 5 pin XLR plugs. Some manufacturers fit 3 Pin ones instead - meaning ordinary audio XLR cables are often used - which mainly work ok, but are not always that good, electrically. Worse still is that some makes swap around the wiring on pins 2 and 3. So most lighting people carry 3 to 5 pin adaptors and adaptors that swap pins 2 and 3.

 

That's the basics. I've skimmed over some bits, but for a newcomer, it will do for the basics.

Paul

[themadhippy]

http://img.photobucket.com/albums/v177/themadhippy/dmxfordummies.jpg

Think Amazon is out of stock at the moment

[LXRob]

Thanks a lot Paulears! and thanks to WOL for letting me now how to use the site properly. Basically I have worked with DMX stuff, martin moving heads, etc and just wanted to better understand the terminology in terms of what it meant practically, a great help guys and I have just bought a really good book on amazon that explains it quite well too.

 

Thanks =]

 

Rob

[Ken Coker]

mmmm....could be time for my legendary Postman Pat animation......

 

KC

[Ken Coker]

Postman Pat does DMX.....sadly, the animation on the last slide doesn't work on Slideshare.!!

Email me if you want the full 45 slide lecture...you'll also find my WYSIWYG lectures there.

 

KC

[boatman]

Postman Pat does DMX.....sadly, the animation on the last slide doesn't work on Slideshare.!!

Email me if you want the full 45 slide lecture...you'll also find my WYSIWYG lectures there.

 

KC

 

I like the analogy, but shouldn't the postman deliver copies of all 512 letters to every house and leave it to the householder to open the right ones?

[Ken Coker]

Postman Pat does DMX.....sadly, the animation on the last slide doesn't work on Slideshare.!!

Email me if you want the full 45 slide lecture...you'll also find my WYSIWYG lectures there.

 

KC

 

I like the analogy, but shouldn't the postman deliver copies of all 512 letters to every house and leave it to the householder to open the right ones?

 

I take your point; in my defence this was the only moment of levity in a two hour lecture...

[Wol]

Postman Pat does DMX.....sadly, the animation on the last slide doesn't work on Slideshare.!!

Email me if you want the full 45 slide lecture...you'll also find my WYSIWYG lectures there.

 

KC

 

I like the analogy, but shouldn't the postman deliver copies of all 512 letters to every house and leave it to the householder to open the right ones?

 

Would a better analogy be 512 pidgeon holes rather than letterboxes? He posts the channel data to the pidgeon holes one by one, and the relevant person (fixture) looks at their set of pidgeon holes (their DMX address). If the fixture wants, it can look at other pidgeon holes, but it generally doesn't care about snooping on others peoples mail.

[indyld]

I quite like Wol's use of pigeon holes as a useful analogy. However I feel that sometimes taking analogies too far, even for brief moments of levity, is a sure fire way to confuse a proportion of already confused students (I don't teach an MSc, BA students are different animals)

 

My lighting system 101, including the very first introduction to control, simply uses students physically taking messages to each other:

 

1 person is the desk, and send out 3 channels of data 1@, 2@ etc

Others are DMX packets, and relay that information one at a time

3 students are a 3 channel dimmer - they each only listen to their part of the packet.

They then move on and "set" their fixtures to one of 3 positions to represent 0%, 50% and Full

 

The extension of this includes a second 3 channel dimmer 4-6 and this can be taken further for more complex fixture, giving each fixture another attribute as well as brightness.

 

The whole activity is so silly that it easily sticks in the mind for terms to come, so that we can quickly return to the "remember when so and so was saying 1@, 2@...." etc. and push the learning further.

 

// edit: Guh, course I totally forgot to mention that On Stage Lighting has a popular and simple explanation of the DMX system. But when I've tried to post useful links in the past, some people complain to the mods :(

[kitlane]

 

My lighting system 101, including the very first introduction to control, simply uses students physically taking messages to each other:

 

1 person is the desk, and send out 3 channels of data 1@, 2@ etc

Others are DMX packets, and relay that information one at a time

3 students are a 3 channel dimmer - they each only listen to their part of the packet.

They then move on and "set" their fixtures to one of 3 positions to represent 0%, 50% and Full

 

The extension of this includes a second 3 channel dimmer 4-6 and this can be taken further for more complex fixture, giving each fixture another attribute as well as brightness.

 

The whole activity is so silly that it easily sticks in the mind for terms to come, so that we can quickly return to the "remember when so and so was saying 1@, 2@...." etc. and push the learning further.

 

 

I've tried a similar excercise. Sitting down was dimmer at 0%, standing up was 50%, standing up with hands in the air was 100%. It didn't work as well as I hoped (I probably made it too complicated by including everything in the packet, MAB, MTBF etc.) Perhaps I'll have another go next year

[Phosphene]

Try http://www.phosphene.co.uk/DMX.htm. Or it's covered in basic form on our DVD 'Light' from www.theeventshop.co.uk I'll answer specific queries, if I can, if you want to email direct.

[Wol]

Try http://www.phosphene.co.uk/DMX.htm. Or it's covered in basic form on our DVD 'Light' from www.theeventshop.co.uk I'll answer specific queries, if I can, if you want to email direct.

 

My eyes hurt..... And now my world has turned green.....

 

I'd be interested to see how many people think that the DMX address gets transmitted down the cable, as so many analogies I've seen seem to suggest that the address is sent (e.g. number on the front of a bus).