DMX

From Ents Wiki
Jump to: navigation, search

Digital Multiplex (DMX or USITT DMX512-A) is the industry standard lighting control protocol, and the protocol used in the union.

Channels

The DMX protocol specifies that, in each Universe, there are 512 addressable 'channels'.

It is standard for each DMX channel to correspond to one attribute. For dimmers, there is only one attribute (brightness), thus these only take a single channel of DMX. More complex lights such as intelligent lights have more attributes that can be changed - for example, Dimmer, Shutter, Pan, Tilt, Colour, etc - and therefore use up more than one channel. Each fixture will be assigned a "start address" - the first DMX channel it will listen for. Multiple fixtures can be set to listen to the same start address to save channels - though care should be used to ensure they are of the same type (or use the same set of channels - this information can be found in the fixture manual or on the fixture's Wiki page).

Each channel has 255 levels. The level is best thought of as "how much you put the fader up", so for example, if channel 1 was for a Profile, 0 would give no light, 25 would be approximately 10% brightness, 128 would be 50% brightness, and 255 would be 100% brightness. On most consoles, these levels will be shown only as percentages (with "FF" or "FL" often meaning 100%).

Universes

A set of devices listening to one DMX signal is referred to as a Universe. The DMX Universe is constructed by daisy-chaining devices together. The DMX Standard instructs us to chain no more than 32 devices together (without a DMX amplifier/booster), though the non-perfect conditions in Venue 1 reduces this to roughly 20.

Multiple universes can be used to allow consoles, such as the Pearl, to control more fixtures than will fit in one universe. A typical use of this in Venue 1 is to attach the Dimmers to one universe and the Optosplitter for the Intelligent Heads on the Octagon on another. In fact, the Pearl allows the use of multiple universes on 1 cable (see the section on cables below).

Terminators

The DMX data path must be terminated at both ends to prevent information from "bouncing" back down the cable and creating interference. One of these terminators is built into the lighting desk, while one must be attached to the output of the last device in the chain.

Some intelligent lights and dimmers have terminators built into them, whilst others do not - if you are not sure, you should attach a terminator anyway.

The terminator itself is simply a 120ohm, 0.5W resistor across cores 2 and 3 (and possibly 4 and 5) of the cable (see below).

Mergers

In the usual setup 1 DMX controller controls multiple different universes of DMX connections. It is possible however to control the same set of lights at the same time using 2 different controllers. This cannot however simply be done by connecting 2 DMX controllers to the same network as this will cause inconsistencies as one controller will think a DMX device is doing 1 thing while the other controller will think it is doing something else. To solve this problem a DMX merger is usually required. This merger allows the merging of multiple input DMX signals (>2) into an output signal depending on well-defined rules such as highest takes priority.

Cables

XLR5 pinouts

There is technically no such thing as a DMX cable or connector. Instead, we usually use XLR cables which are usually thinner (and of lower resistance) than the ones we use for sound. Convention tends to be that we use five-pin XLR, or XLR5, though many manufacturers use non-conventional connections (see below).

Typically, DMX cables have three cores: one ground (Pin 1) and a twisted pair bus with one hot (+ve, pin 3) and one cold (-ve, pin 2) wire. Pins 4 (-ve) and 5 (+ve) are a spare bus and are sometimes used to carry a second universe on the same cable. (The latter two cores should be used with care - for example the Pearl may use this second universe feature, but some manufacturers have used these cores for power.)

Technical Information

Each bus uses the RS-485 serial protocol which must be terminated at each end.

The lighting desk sends up to thirty packets of 512 bytes in length each second, allowing for thirty updates of the lighting state per second. Each of the 512 bytes is a DMX channel and are numbered from 1 (first byte sent) to 512 (last byte sent). Each channel has 256 values (0-255). Some devices use 16 bits for pan, tilt, etc: these are sent using two DMX channels.

Non-standard connectors

Some equipment does not conform to the standard XLR5 connectors, instead using XLR3 or XLR4 connectors.

XLR3 connectors normally use the same pins as XLR5, except for most Martin fixtures (518s, 812s, 1220s, etc), which have swapped the purpose of pins 2 and 3 (i.e. cold and hot) for historical reasons. To use these Martin fixtures, a pin-swap is needed.. These can be found in the Ents Cupboard. Note that, unless the only fixtures in the chain are Martin fixtures, pin swaps need to be used in pairs - one before the fixture to swap the signal to Martin-mode, and one to change it back for other fixtures.

XLR4 is used for the scroller units. A control box above the Octagon converts the XLR5 system to the XLR4 system for the scrollers. The scrollers are connected in a ring (using XLR4 cable) back to the control box.

A summary of DMX connectors in our venue is given below:

Type Pin 1 Pin 2 Pin 3 Pin 4 Pin 5
DMX standard (XLR5) Ground Data -ve Data +ve Spare -ve Spare +ve
XLR4 (Scrollers) Ground Data -ve Data +ve Power +24V -
XLR3 (except Martin) Ground Data -ve Data +ve - -
XLR3 (Martin) Ground Data +ve Data -ve - -

Problems

  • DMX is a one-way protocol: the lighting desk sends instructions to the lights and no information is sent back. Thus the lighting desk does not know if the lights have power, are working, etc..
  • The packets on each universe do not carry any information on channel function nor universe number, thus lights/functions will obey the instructions on their channel even if it is not for them. (E.g. if pan for light 1 is on channel 10 but channel is set to dimming on the lighting desk, then light 1 will pan as the fader for the dimmer is raised and lowered!).
  • RS-485 (serial protocol underneath DMX) is a bus system. This means that all fixtures read the same signal regardless of position on the bus. Hence lights can be placed in any order on any DMX universe.
  • DMX has no error correction: thus any interference will not be detected by the fixture. Poor cabling will increase this interference, causing the lights to behave erratically or fail to respond completely. For this reason, "critical" controls, such as for moving rigs or pyrotechnics, should never be controlled using DMX.
  • DMX is a multiplexed system: unlike a serial system which may only send data when the lighting state changes (i.e. if a fixture misses a state change then it will hold the previous state until the next state change), DMX constantly sends the state up to thirty times a second. If the link from the desk to a fixture is broken, then the fixture will usually hold the last state. Beware that as soon as the link is restored, the current state will be read by the lamp, even if it is completely different. (A useful note: if the lighting desk fails, then unplug the cables from the back to hold the present state, restart the lighting desk, set the state and then reconnect the cables. This will prevent a black out when the desk is reset.)