What and how to connect – the mechanics of DMX
So now you have purchased or built your own desktop DMX lighting controller, what next?
The 1970s glitz and glamour model
If one looks back to the 1970s and the disco scene there was a lot of glitz and glamour to spinning mirrors, strobe lights, colour organs and mood lighting in general – don’t forget the silver jackets. UV or black tube magic was very popular. Don’t forget the gorgeous girl with dark underwear. This was all just very kitch. Nowadays “clubbers” wouldn’t be seen dead in a place that sported such grotesque lighting but do bear in mind this was just a phase mom and pop went through. Coming into the millennium there were some surprises in store: 3D laser – holograms, motor driven lighting, pan, tilt, high powered strobes (we had that one before) – all controlled from one interface, either software driven from your computer or a free standing light mixing console.
Mechatron your idea
Along with the Arduino, Pi and Bone we have had some interesting changes in buyer focus – 3D modelling, CNC and drones. Why the importance?
Technically speaking any form of sophisticated robotics 30 years back was unaffordable for the hobbyist. Not so now. H- and Half Bridge high powered motor drivers, high torque stepper, servo and low voltage AC/DC motors, hardware and software controllers, gears, pivots, bearings, threaded and linear shafts and aluminium extrusion are all available at your local electronics shop. Mechatronics is now in fashion – electronics students now study mechanical engineering and vice-versa. DIYers are becoming experts at amalgamating the mechanics used in stage lighting shows to sophisticated control. And this is happening right now.
A DMX512 controller and lighting display of considerable sophistication will set one back R10 000.00 in South Africa. Now, the home enthusiast has no boundary as to what he or she can build. As is so often the case ready to purchase products are cheaper than what one could make but we emphasise innovation here. Much of what we do to stage lighting could be used to driver cameras as well.
But let us move on to how it all works
The DMX protocol uses serial communication. Each receiving device would be set to a specific channel number or address e.g. your primary light colour organ would have settings 001, 002 and 003 for RGB. You may have 4 of these arrays on a stage all triggered from the same source i.e. RED set to 001, BLUE 003 and GREEN 002. The beauty about this setup is that the cabling runs in series as well, all daisy-chained. In-Out. When channel 001 triggers all the red lights will be activated. This is commonly known as the DMX Universe which can utilise all 512 channels. There are caveats though. In large lighting displays these Universes are wired to work in conjunction with each other.
A channel can have up to 255 attributes. This should ring a bell – it’s binary of course. If you do have excel at home: =DEC2BIN(255) = 11111111 or reverse =BIN2DEC(11111111) = 255.
A control on one light has therefore 255 permutations, faders, pan and tilt, linear horizontal and vertical tracking plus a multitude of other functions.
Although the DMX512 protocol allows 512 channels only 32 channels should be used daisy chained together on one cable run due to the line impedance i.e. voltage drop and data loss. Professional lighting engineers stick to short runs, possibly of up to 200m or 600ft. There are DMX repeaters also known as opto-splitters available which allow the rigger to drive a further 32 devices.
The usage of DMX is infinite but…. what’s the issue about cable type?
As one can imagine DMX can be used for almost anything where an element of control is desired. Of course it can now be done via wi-fi as well. All very ingenious but there is an element of confusion arising as to which cable to use. Professionals stick to proper DMX cable with 5 pin plugs and sockets. Not because the standard prohibits 3 pin XLR but because sticking to standards relieves one of the stress later to find bizarre switching taking place through data loss or worse, corruption. Your DMX cable is 120 Ohm, your XLR audio cable has an audio frequency spec, a lower impedance. This drop in impedance which is due to the inductive and capacitive properties of audio cable is not good for DMX which is strictly data transfer. And yes, CAT5 cable is often used with great success in fixed stage lighting. Be careful of interference and no shortcuts. I.e. use conduit and trays – preferably not poly-saddled to fixtures.
Servo, Stepper Motors and Bi-Directional control
Servos and steppers have long been used in aviation and maritime for their accuracy to drive and relay angular movement but their pricing was prohibitive to casual buyers for experimentation. Times have changed. Even pricing for radio control equipment for hobbyists has dropped to the point where experimenters are purchasing for prototyping in other fields of interest.
Both servo motors and stepper motors need a feedback system of sorts for resetting or alignment. Synchro mechanisms include pulse generators and receivers. On a ship the gyro compass north up position is sensed, amplified and feeds a synchro system which drives all the repeaters e.g. possibly monkey island, bridge-wings, manual steering gear room and of course, at the helm. Auto piloting needs a feedback system which applies rudder for correction. However your standard legacy DMX512 system did not allow for remote monitoring which made it’s use in mission critical systems unsuitable.
In a DMX, system control is carried forward from user to device and there is no feedback system – in other words there is no synchronisation for monitoring. In simple terms, if a camera is set to 45º or say NE exactly from a console there is no way that the operator in fact knows that it really stops at NE, a degree here or there in some instances is a huge error. The path of the feedback system could include a synchro generator with an optical or magnetic switch set to exactly 0º or North as the alignment sensor. When trimming the controller moves the camera through 360º and at exactly 0º a difference signal between camera position and console setting (of 0º) is generated which applies correction either automatically (preferred) or through a trimmer adjustment. Of course this is seen when our RC aviation hobbyists trim the rudder.
NE when it The ANSI E1.20 standard allows for RDM or Remote Device Management over DMX512 networks. RMD devices are very expensive.
Whilst the idea of building one’s own DMX controlled device is inviting often the cost is more than that of a commercially available alternative. Like most things electronic we have plenty of enthusiasm but the end result, the cosmetics leave a lot to be desired. If only there was a metal working shop around the corner that could build every design under the sun at the drop of a hat and cheaply!
I have never built my own DMX controller neither a DMX512 controlled device but have had a bit of experience in the setting up of systems for DJ usage. In time DMX will prove to be very popular in home automation because of the strict communication protocol.
But… many kudos for those that did and here’s some interesting links
Remote controlled Pan and Tilt head – the mechanics
USB DMX Dongle for $10 – Really great USB-RS485 controller
8 Channel DMX dimmer pack – using a ATmega8515-16PU and 75176B Tx/Rx
8 Channel DMX switch pack – as above, up to 160A switching possible.
DMX Projects – Controllers, relay operation, DMX to analogue (purchase only)
Note from Admin: Although there is a lot of interest from the DIY community with regard to DMX control the one thing that does stick out like a sore thumb is that build cost is often more expensive than buying a commercial device. Often the home built unit does not fulfil DMX protocol and commercially added devices don’t function properly or at all on these controllers.
South Africa has quite a few retailers with online presence selling DMX based controllers and devices – take a view at what is available and pricing before embarking on an ambitious product. Buying a cheap motorised device and adapting it for other uses is often just as rewarding. You can even call it your own! 🙂