How to talk to your dimmers

The signal between the control desk and the dimmer rack can be one of four things:

1) Analogue Voltage Control
A control signal of 0-10 volt DC variable voltage is sent from the control desk to the dimmer racks via a multicore, (the control cable), using one wire per channel, a common return and a technical earth.
This variable voltage corresponds to the position of the channel fader / master combination on the desk, and is used directly to control the output from the dimmers.
There is no agreed pin wiring protocol for final connection, so various manufacturers control cables and desks are not necessarily compatible with each other. Other manufacturers may use a different voltage.
A Control voltage of approximately 18V DC is derived from the dimmer racks and fed to the desk where it is controlled by the fader system to produce variable voltage of ( typically) 0-10 volts sent to each seperate channel via its own core in the control cable multicore.The dimmers interpret this variable voltage and use it to control the dimmer's output.

This stands for Digital Multiplex, and is a variant of the normal protocol by which computers talk to each other. The idea is that each channel in the control desk sends its information in turn down a single cable to suitably equipped dimmer racks or other equipment such as scrollers, moving lights or other DMX-compatible items. This process is called time division multiplex or, colloquially, MUX.

The first piece of information that is sent is the channel destination address, which alerts the particular device to expect instructions. It is then followed by the instruction itself, which could be dimmer level, fade or movement information and finally some checks and a stop instruction.
DMX signals may be looped from item to item. Up to 512 channels can be sent down a single DMX line. If more channels are needed then a second line can be used.

At the receiver, a device called a demultiplexer or demux decodes the serial signal into up to 512 separate channels and distributes the signals to the correct destination. Each receiver will be set to a number which is its identity, and will then respond only to instructions sent to that number. A dimmer rack or moving light using more than one control channel will usually have its own demux which is set to the correct start number, the remainder of the numbers in sequence being automatically distributed. Individual receivers such as smoke machines may be equipped with their own built in demux.

Start numbers are set either digitally or by means of thumb switches. Sometimes you have to set the numbers in binary code.

Many demux units on the market also contain backup memory systems, so that in the event of failure of the DMX signal, the existing lighting state is retained in memory. In some units, additional memories are available.
Modern units which use more than one control channel, (for example the Strand Hyperbeam uses seventeen) create their own problems: For example if you have ten of these units they would use up an impossible 170 control channels- a whole lighting desks worth! Then you have the problem of trying to remember what channel for example, the iris on unit number nine is, or the cyan filter on number 4.
This is solved by the use of Attributes. In this system each unit is allocated a single control channel number, and the internal patching in the control desk automatically distributes the remainder as decimal points of the main number. Thus the main control channel may be, say, 34, in which intensity might be say, Channel 34.01, the dowser 34.02, the iris 34.03, the colour wheel 34.05 etc. In a properly set up system, you would allocate intensity of each unit always to be attribute .01 and all the irises on .03 and so on, making it fairly easy to remember.

Emulators are available, to simulate any output required for test purposes, which can also act as DMX cable testers.

CONNECTION: DMX cables must be suitable for E1A485 (used to be called RS485) with one or more low-capacitance twisted pairs with overall braid and foils shielding.
Suitable types have a 100 - 120 characteristic impedance. Conductors should be a minimum 24 AWG (American Wire Gauge) or 7/0.2. sq. mm.
Standard microphone cable is not usually suitable!

The preferred connector type is an XLR-5 pin: The pin wiring details are as follows:

PIN 1	0 Volt ground/return
PIN 2	DMX- usually black
PIN 3	DMX+ usually white
PIN 4	Spare - (usually green) 12-24 Volts DC (where required)
PIN 5	spare + (usually red)
CHASSIS	NOT CONNECTED

Some early equipment may be fitted with XLR-3 connectors. This is not standard and does not form part of the official specification for DMX. In this case the pinout is the same as the first three pins of the XLR-5.
Some scrollers use 4-pin XLR, pin 4 carrying the scroller operating voltage. the pin-out being the same as the first four pins of the XLR-5. Special cable is available, for example that supplied by Rainbow Scrollers, which carries one twisted pair and one power pair.

Normally, all items are daisy-chained, with the console at one end of the line and the last item in the chain terminated with a propriety terminator to avoid unwanted digital reflections. This consists of a 110 quarter-watt resistor across pins 2 and 3 of the XLR . The system may well appear to work perfectly well without one, but is likely to fail unexpectedly at an embarrassing time.
Some items may have a built in terminator switch, labelled something like “last rack” or “end of line”. This should be switched off except where it really is the last item.

NETWORKS
Where a complex installation is envisaged, a NETWORK can be used
This consists of a backbone cable from which may be tapped DMX signals as required, using an input module at every point at which you may wish to plug a controller, and an output module at every point that you wish to connect a fixture.
Up to 32 modules may be used on each line. These completely optically isolate each fixture on the system from each other, none of the above fault conditions can cause more than the loss of a single fixture. If two desks are plugged in at the same time, the one physically nearest to the smart splitter or the first fixture takes control, and the other is ignored.
A 12-24 Volt DC supply is required at each module, and this may be derived from a SMART SPLITTER. This accepts the primary DMX signal from the desk and splits it into a number of lines or legs which may be sent as separate backbone cables to different zones or areas. One such zone may be the dimmer racks, another, moving lights, a third the scrollers FOH etc. Under this circumstance, a fault of one leg cannot affect anything except other fixtures on that leg and so damage limitation is possible. It is unnecessary to terminate the end of the line where a network is used.
Hard wired Y-Splitters or “two-fers” will not work, and will cause severe signal degradation and peculiar signal reflections.

If more than 32 devices are required on one line or splitter leg, then a repeater amplifier may be used. It is important to note that these are amplifiers and will only amplify what is there, and will not re-create missing data.
Systems are available which will clean-up dirty DMX, converting various manufacturers versions of DMX if needed.
It is possible to use two controllers simultaneously, each controlling a different part of the DMX spectrum. This is done by the use of a DMX SMART MERGE BOX.

Unfortunately there is more than one variant of the DMX protocol, and they are not fully compatible. Variations include:
PMX (Pulsar Multiplex)
D54 Multiplex
SWC (System Wide Control)
ShowNet

a)         A very long daisy chain is used,
b)        The end of the chain is not properly terminated,
c)         A faulty or unsuitable DMX cable is used,
d)        If two DMX sources, for example a lighting desk and a backup system try to use the system at       the same time,
e)         If an individual moving light, scroller etc. is faulty.

A fault means that everything grinds to a halt. However, the use of a network can usually confine the problem to an individual fixture.

3. ETHERNET
By expanding the concept of a lighting desk to include networks of input / output devices rather than a single, complete device, the result is a communications system. If the memory unit and arithmetic and logic unit of a computer are both relatively small but control many input / output devices, then the computer will act more like a message-handling system than a computational device

The ETHERNET SYSTEM is widely employed in the computer industry to create these networks, and is rapidly coming into use in the theatre. Individual network stations, which can consist of any suitably-equipped lighting desk or moving light system, etc. are connected via "nodes." Nodes are connected by coaxial cables, optical fibres, or twisted pair cables. Networks that are set up for individual functions and purposes are called local-area networks, or LANs. Wider networks are possible, however, and use telephone lines to connect to other systems. and are known as WANs (Wide area networks). Local-area networks are looped around the building and are simply ‘teed off’ into an Ethernet Node as required. The vastly increased speed and bandwidth has made possible a huge increase in the amount of Data, (at present more than 1536 separate DMX signals), which can be sent down a single cable and also increased the distance over which the signals may be reliably distributed.
Experiments in various lighting companies in such areas as voice recognition, digital glove input software and touch screens point to an important future for networked computers in stage lighting.

CONNECTION:
There are two major wiring systems:

Co-axial links ( legacy wiring - out of date now) may be used using 50 Ohm coax cable. Standard 75 ohm video cable is not suitable.
The cable is run from the control desk to each of the places where a connection is, or may be, required. At each point the cable is terminated into a BNC connector which is plugged into a proprietory Tee plug which in turn plugs directly into the node associated with the item, stalls controller, video screen, etc.The circuit is continued from the other branch of the tee to other positions as needed.

Twisted Pair links require the use of a network hub which is basically a splitter module. The hub is connected directly to the lighting desk and the outputs from this are taken singly to the individual nodes, or further hubs.

The correct cable is Category 5 4 pair UTP
The connectors are the RJ45 telephone type.

Modern Theatres have an extensive networked sytem. The Royal Opera House, for example, a network of Personal computers linking the entire premises, from the box-office to the fly-floor, dealing with all the day-to day work of a busy theatre office, as well as holding flying, lighting, sound and hanging plots, rehearsal schedules and calls, as well as keeping track by means of light pen and bar code of scenery and props. For the historian, 35 years of archive are available. The network allows access to various parts of the system only by authorised persons. At present there are no plans to link the system to the stage lighting board, but...watch this space

4) MIDI
This stands for Musical Instrument Digital Interface. As you might gather this was invented for the purpose of interfacing musical Instruments, so many of the terms are music-oriented, such as velocity zones, note-on note off etc.
In a musical keyboard, each note and key press together with information relating to key pressure, sound envelopes etc., generates a unique code, which can be exported to, and saved to a sequencer.
A lighting control can be made to do this as well. A sequence of actions on the desk generates a sequence of codes. For example, the Sequence involved with initiating a cue, say Q34, goes as follows:
PRESS “Cue No” button- PRESS button “3” -PRESS button “4” -PRESS button “Transfer to playback A”- Press “Go”

The sequencer can also simultaneously record information from other sources, for example a sound track or a MIDI equipped projector controller. Once recorded onto a sequencer, the show may be edited on or off-line using a graphical digital editor supporting cut-and-paste, etc.

MIDI can also be used to extend a desk by allowing its masters to operate a slave desk. It can also be used to make one desk slavishly follow the actions of another, allowing an instant backup system in case of desk breakdown.

Any MIDI keyboard or other MIDI-equipped device can operate a lighting desk. Alternatively, a lighting desk can play music!

Midi can enable a fully integrated show to happen very easily, incorporating any MIDI-capable item to be interfaced, such as slide and movie projectors, sound desks, moving light and scenery controllers and of course synthesisers etc. This is a serial signal, so only one event can happen at a time, but events can be spaced apart as little as one twentieth of a second.

MIDI instructions are sent as a series of of bits. The transmission rate is 31.25K, with each data word consisting of 10 bits. Each group of bits making up an individual MIDI code is called a “Message”
There can be up to sixteen channels of MIDI active at once, each carrying separate message streams, and so systems can be stand-alone, or form part of a larger system. Midi may be daisy-chained from item to item using a single cable between each. MIDI-equipped equipment has at least two sockets, possibly three, using standard 5-pin 270? DIN connectors. These are:

MIDI-in          for receiving MIDI from other devices
MIDI-out        For transmitting information
MIDI-thru       this simply acts as a splitter and passes the signal along unchanged.

MIDI cables are connected with 5-way screened cable joining like-numbered pins, with the screen and one core on PIN 1 and the others connected to the remaining pins. Standard Hi-fi cables work perfectly well, but a limit of 40 feet is advised. Check that it is “straight wired” not mirror image wired.
Care must be taken when connecting equipment, as 5-pin DIN connectors were used on some older equiupment for different purposes, such as “sync” between rhythm units, and a mistake could result in expensive repair bills or more likely nothing happening.

MIDI relay boxes are available to allow a MIDI signal to operate, or be operated by external pieces of equipment. For example, using the M+M MIDI Box, a door switch or pessure mat can be made to operate a sound or lighting cue. For example, a microswitch on a fake gun can operate an infra-red beam which is connected to MIDI receiver triggering a gunfire effect. thus ensuring perfect self-operating synchronisation. more about MIDI

The latest development is MSC (MIDI Show Control). This simplifies the codes to allow more information in a smaller time space, transmitting complete instructions rather than individual key presses. In the example above, the string would be replaced by “GO Q34”

Midi is not directly compatible with DMX, but can be used in conjunction with it using a specialist interface unit.

Unfortunately there is little cross-manufacturer compatibility. For the complete MIDI show control protocol and standard click here

Strictly, this is not a control protocol, but is included as a method of using one piece of equipment to control another.

SMPTE (pronounced ‘simpty’) stands for Society of Motion Picture Theater Engineers This is a system whereby a numerical code is inserted onto a film track or onto recording tape or stored as a digital datastream

It is simply an electronic count which is produced by one of the pieces of equipment or from a stand-alone synchroniser, producing a continuous stream of numbers starting from any number you care to set it at and increasing by one every thirtieth (USA) or 25th (UK) of a second. This number is chosen to represent the frames of a cine camera, and each sequential number is therefore known as a frame.

It is normal to use an 80-bit system: The first eight bits represent Hours: Minutes: Seconds: and Frame No: it takes the form of say 06: 42: 20: 19 or six Hours 42 minutes, 20 seconds and 19 frames. The maximum count is 23: 59:59:23
The next series of bits of variable length are used for user data, and can be used to represent say, reel number, or any other information. The final bits give directional information which tells the reader which direction to read the bit in, so that the information can be read from either direction: this way you can even read the information whilst in fast rewind mode.

This can be used as a synchronisation system, whereby each device, be it a lighting desk, a sound source, camera, motion control system, laser system or pop-up toaster, anything fitted with a decoder and relay set, can be made to operate at a specific time. The signal is simply sent digitally around the system and tapped off where required. It enables live shows to be synchronised as well as easing pre and post-production editing. The decoder simply waits for the correct Frame Numbers to which it has been preset to come along and operates a relay or similar device.

On a lighting desk, it would be normal to set the playbacks to auto-sequence and allow the SMPTE signal to initiate the GO button

It makes planning a show very simple as the whole thing can be mapped out on paper or spreadsheet first and then use to create lists of events which can be sent out well in advance to the various effects contractors

THE INTEGRATED SYSTEM
Systems are available from more than one manufacturer which enable you to interface all the control protocols above, and control them from a single terminal of keyboard. ShowCAD from Axon digital design is such a system, and allows for simultaneous control of MIDI, DMX, Analogue, SMPTE more about SMPTE