Introduction
The first stage-lighting system I ever used was installed at Irving Crown H.S. in 1964. The dimmers (there were seven) were huge autotransformers in a six-foot by eight-foot cage in the stage-left wing. They were operated by levers - there was no such thing as remote control on the school district's budget. A better-financed theater might have some degree of remote control: the levers could be replaced with motors and the motors operated by pushbuttons at the remote station.
The invention of the SCR (Silicon Controlled Rectifier) and the advent of solid-state light dimmers meant a big improvement. The six-foot by eight-foot cage became a nineteen-inch rack. An SCR allows a large amount of power to be controlled by a small voltage (usually 0-10 volts) - instead of needing a huge lever, each dimmer could be controlled by a small fader. Remote control wiring was simplified: Instead of needing a motor to operate the fader, the fader itself could be moved away from the dimmer. Instead of needing 3 heavy-gauge wires to control a 120-volt motor, each dimmer needed just two wires for the fader - and they could be pretty small, because they only handled the 0-10 volt control signals.
This 0-10 volt analog system is still in use today in many places. But smaller dimmers meant that you could put more of them in the same place, and systems got bigger and bigger. More dimmers meant more faders... and more wires between the controller and the dimmers. Multi-conductor cable is expensive, but in the seventies, digital integrated circuits started to get pretty cheap, paving the way for multiplex systems.
The idea behind multiplexing is that several dimmers and faders can time-share the same wires. Instead of having 2 wires per dimmer, we just have a single microphone cable from the lighting desk to the stage for all the dimmers. Each fader/dimmer pair takes a turn using the microphone cable for a short time, and the dimmer remembers its control voltage until its next turn. It's still 0-10 volt analog, but with timing information included.
I honestly don't know who came up with the analog-multiplex scheme first: It was introduced by several different lighting manufacturers at roughly the same time. I analyzed several similar systems (I've found no published specifications). Here's how one of them (MUX-64, from James Lighting) works:
Hardware Implementation
MUX-64 devices connect using standard low-impedance microphone cables with 3-pin XLR connectors. Pin 1, connected to the cable shield, is both signal ground and power-supply common. On pin 2, MUX-64 dimmer packs send 15-volt power back to the controller, removing the need for the controller to have it’s own power supply. Some controllers are equipped to accept a "wall-wart" supply as well. Pin 3 is used for the actual control signal, which alternates between a negative voltage for synchronization and a positive voltage for dimmer control.
The signal
The typical MUX-64 controller will send out a negative10 volt pulse of at least 1 millisecond (1000 microseconds) duration to start a data frame. This "long" pulse tells all the dimmers that the next positive signal belongs to dimmer #1. The control signal then moves to a positive voltage in the range 0 to 10 volts, corresponding to the brightness expected from dimmer channel 1. It holds this voltage for a period of 150 to 200 microseconds, to allow dimmer 1 to sample the voltage, then swings back to the negative 10 volt level for a period of 100 to 150 microseconds.
This short "sync pulse" tells the dimmers "okay, now we’ll do the next channel." It’s followed by a 150-200 microsecond positive voltage corresponding to the brightness expected from dimmer 2, then another short "sync pulse" and so on until control voltages and sync pulses have been sent for as many channels as the controller can handle. The process is then repeated, starting over with another long "sync pulse."
Each dimmer pack has a counter, used to count "sync" pulses. It compares this count with its address switches - when they match, the dimmer collects the next control voltage as its own.
Here's what it looks like on an oscilloscope:
In the above drawing, channel 1 is off, channel 2 is at 25% brightness, channel 3 is at 50% brightness, channel 4 is at 100%, channel 5 is at 50%, channel 6 is at 75%, channel 7 is at 100 % and channel 8 is at 50%.
There were minor variations in the duration of the long sync pulse and the duty cycle between short sync pulses and control voltages between manufacturers, but the major difference was simply the maximum number of dimmer channels controlled. Sunn started with 32 and stayed there. James Lighting started with 64 and stayed there. NSI started with 64, then later expanded to 128. Leprecon adopted NSI's system. Lightronics started with their own incompatible variation, then moved to copy NSI. Note that this is the maximum number of channels the manufacturer will support on their largest controller. Very few controllers actually approach the maximum. The most popular multiplex controllers handle 16, 24 or 32 channels.
Some caveats about mixing various brands in the same system:
Sunn and ETA dimmer packs expect no more than 32 channels of information. If given more information the address counter rolls over, so that channel 1 on the dimmer pack responds to channel 1 data AND to channel 33 data, channel 2 responds to channel 2 data and channel 33 data, and so forth. This is why I had to create a special version of the DMX-lator I just for Sunn and ETA dimmer packs, limiting the output to just 32 channels. This version will also operate James Lighting, NSI, Leprecon and newer Lightronics dimmer packs, but only when assigned to the first 32 channels.
Very old Lightronics dimmer packs used variations in the sync voltage, rather than pulse duration, to differentiate between the reset and sync pulses. Reset pulses are at -10 volts, sync (channel-count) pulses are at -5 volts. They also handled a maximum of 40 channels, and sent 12 volts AC, rather than 15 volts DC, back to power the controller. They should NOT be intermixed with the others under any circumstances. The standard version of the DMX-lator I senses whether the power supply is DC or AC and adjusts the sync method and channel count for these old Lightronics dimmers.
MUX-64 dimmers from James Lighting, as well as older
NSI and Leprecon dimmers using the 64-channel version of Microplex, may
operate erratically with newer Microplex-128 controllers, again because
they don't know what to do with the extra channels.