DMX

Digital Multiplex (DMX or USITT DMX512-A) is the industry standard lighting control protocol, and the protocol used in the union. The standard only specifies one type of connector (XLR5), although other non-standard connectors have been used by many lighting manufacturers (see below).

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 may be used for a second universe on the same cable. (The latter two cores should be used with care, for example on the Pearl outputs, as some manufacturers have used these cores for power.)

Each bus used a RS-485 serial protocol which must be terminated at each end, one of which is inside the lighting desk (or DMX splitter). The terminators are required to stop the signal bouncing back at the end of the cable (and interfering with the signal). 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.

Each universe is often a cable run. For example, a common configuration in Venue 1 is for Universe 1 to be the Dimmer Racks, Universe 2 to be the Intelligent lights on the octagon and above the floor, and Universe 3 for anything on stage. Each universe could link to several fixtures: the standard allows for 32, although the non-perfect conditions in Venue 1 reduces this to 20 (without a DMX amplifier/booster).

Each fixture will have a DMX Address. This is the first DMX channel that it reads. The number of channels it reads depends on what mode it is set to, and how many channels that mode takes up. For example, an Alpha Wash takes up 17 channels. If it is given address 10, it will read channels 10-26. (N.B. The next free address is 10+17 = 27.)

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, Gobo, Gobo Spin, Prism, Strike/Reset/Lamp Off, Frost, etc. What each channel corresponds to can often be found in the fixture manual

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 degrees would be aprox 10% brightness, 124 would be aprox 50% brightness, and 255 would be 100% brightness.

The Pearl has 4 main DMX outputs that we use in two pairs of XLR5 outputs (1&3, 2&4). This gives out signal on all 5 pins (i.e for 1&3, the first bus is universe 1, the second bus is universe 3). Since our 5 pin cable is mostly 3 core, to use Universes 3 or 4 requires use of a DMX Y-split. This Takes one XLR5 5-core input (from the Pearl), and gives out two 3-core XLR5 DMX outputs (one for the first bus, one for the bus). For one output, it will put cold (pin 2) and hot (pin 3), onto cold (pin 2) and hot (pin 3), for the other output, it will put cold (pin 4) and hot (pin 5) onto cold (pin 2) and hot (pin 5).

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 Martin (518s, 812s, 1220s, etc.) fixtures which have swapped the purpose of pins 2 and 3 (i.e. cold and hot). Therefore, a pin swap is needed for each change of Martin fixture. These can be found in the Ents cupboard. Note that pin swaps need to be used in pairs - one to change the signal for the Martin fixture, one to change it back for the other fixtures. Sound cables also use XLR3 connectors: these should not be used in place of genuine DMX cables as they do not have a twisted core. Similarly, DMX cables should not be used in place of sound cables.

XLR4 is used for the scroller units. A control box above the Octogon 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|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. Also, moving rigs or pyrotechnics should not 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 minute. If the link from the desk to a fixture is broken, then the fixture will hold the last state, but as soon as the link is restored, the current state will be read by the lamp. (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.)