ETC Dimmer Doubling (Multiplexing)

The ETC site has perfectly good information on dimmer doubling, but I think there are a few questions that come up frequently enough to discuss them here. If you don't know what multiplexing or "dimmer doubling" is, go look at their pdfs (linked below) and come back. The short explanation is that it's ETC's method of controlling two lights (or sets of lights) on two separate control channels while powering them from the same dimmer over the same 3-wire cable.

ETC Dimmer Doubling Datasheet.pdf
ETC Dimmer Doubling User Manual.pdf

ETC calls it "Dimmer Doubling", but everyone I know refers to it as Multiplexing. Please note that on this page both refer to the same thing, but have nothing to do with multiplexed control signals from NSI or Leprecon, etc. This is all about manipulating power to the fixtures for additional control.

What fixtures can multiplex? All ETC Source Four fixtures that use HPL lamps can multiplex. The power connector will have to be changed to an L5-15P (don't use adapters!), and 77 volt lamps will have to be installed. The 77V lamps have a roll-pin on the bottom (90 degrees from where the pin is on a 750W/115V HPL) to prevent them from being accidentally used in fixtures supplied with 115V, and a small screw in the cap will have to be removed to make room for the roll pin. I have seen castings of Source Four 750W caps that had a depression where the 77V safety screw should be; these have to be drilled out to allow multiplexing. I believe this was a brief manufacturing issue, now long resolved.

What dimmers can be used? ETC Sensor dimmers with CEM or CEM+ control modules are the most common. Through the CEM you can assign which dimmers within the rack are multiplexing, and which are not. This is helpful if you have a 2K fresnel in the middle of a position full of multiplexing units. ETC Sensor dimmer packs with MPE control modules can be used, but it is an all-or-none choice. DIP-switch #4 in the ON position tells all the dimmers in the pack to multiplex, and in the OFF position none will multiplex. Less flexible, but cheaper - especiallly if you already have an MPE. MPEs have now been replaced by CEM+ modules, which may require a teensy bit more effort to enable multiplexing.
Both regular D20 and Advanced Feature D20AF dimmer modules will multiplex. I'm pretty sure some L86 dimmer racks can multiplex as well (all-or-nothing like the MPEs); it may be a dipswitch setting, but I don't have the info immediately at hand.

What about cable? The same 20A 2P&G stagepin or L5-20, 12awg jumpers and multicable you're familiar with are used to carry the power to the dimmer doubler. After the dimmer doubler, use jumpers with L5-15 connectors.

How many fixtures can I put on a multiplexed dimmer?
The "how many fixtures" question always depends on the wattage of the lamps, whether you're multiplexing or not. The answer is the same too, with one qualification - each side of the dimmer doubler will only support half of the dimmer's maximum load. So you can plug two 550W 77V lamps into each side of the dimmer doubler for a total of four fixtures, but shouldn't plug two 750W 77V lamps into the same side of the dimmer doubler no matter what's plugged to the other side. You probably aren't planning to multiplex your Multipars. If you did, you'd want to make sure you're not trying to pull too much power on one side of the Dimmer Doubler, which would be likely with higher wattage lamps and/or longer strips.

Does Multiplexing save money? Complicated question. You save money by using potentially half as many dimmers and half as much cable, but you have to rent or purchase the multiplexers (Dimmer Doublers). It can certainly save you room on the truck if you're carrying half the dimmers and cable, and it could potentially save you weight on a position by requiring less cable. You may save labor in running only half as much cable, but you may increase labor if the crew is not familiar with how it all goes together. If you're also using 115V fixtures, you'll need to carry spare lamps and caps for both types. If you're unpgrading an installed lighting system, multiplexing can save a lot of money - you may be able to double the number of circuits available to hanging positions without replacing/adding wire.

Does Multiplexing use less power? Nope. It's a nice dream, but would equate directly to getting something for nothing. You get more or less the same light output for more or less the same power input as a non multiplexed dimmer. The benefit is in additional control - multiplexing can turn your 48x 2.4k rack into a 96x 1.2k rack - effectively twice the number of dimmers at half the capacity, with the same power consumption.

If they're in the same dimmer, how do I patch them for separate control? On current ETC consoles with current software (and some older consoles depending on the software), you simply identify the dimmer as set for multiplexing, then you can patch dimmer 3A and 3B separately just by typing the appropriate letter after the dimmer. On consoles that don't understand the letter identification of multiplexed dimmers, patch the "A" side of the dimmer as the actual number, i.e. "3", and the "B" side as 256 dimmers higher, or "259". This way you can use your Microvision FX (or any other DMX console that can address 512 dimmers) to control multiplexed dimmers. It's just hard to remember that dimmer 137B is really patched as dimmer 393.

If dimmer 3B is really dimmer 259, where's dimmer 259? Because each multiplexed dimmer requires two DMX addresses (A and B or X and X+256), and DMX512 only allows for 512 addresses, only 256 multiplexed dimmers can be controlled per DMX universe. The 259th dimmer in the rack would be the third dimmer in the second universe. Note that at least in some versions of Obsession software, multiplexing the first DMX output would limit the second (and possibly the third?) outputs to 256 dimmers each. The solution is to control the multiplexed dimmers from the third port, and address the moving lights, etc. though the first two ports. I'll try to pin down the exact version of software and port configuration and post an update... but don't hold your breath.

Any questionable practices to share? I don't know where ETC stands on these, so let's say they're not recommended just to be sure. You can plug a regular 115V fixture into a multiplexed dimmer without a multiplexer, and patch both the A and B dimmers to the same channel, and it will work normally. This is cool for when you've multiplexed your whole 24 pack on the MPE and have that one non-77V fixture. Similarly, you can plug a 77V fixture into a dimmer doubler, and plug that into a regular dimmer, and it too will operate normally. Now that I think about it, it flickered a bit on the Dilor rack, but what are the odds someone else will use multiplexing fixtures on a Dilor rack?

One more note: The 77V HPL has a smaler filament than the 115V lamps, which makes lamp focus that much more important. Source 4 Pars have no provision for adjusting the lamp, which could mean that lamps from different manufacturers may look different with some lenses - but only if you're really Peggy picky. Either way, focus carefully.