DMX 512 Protocol Standards 2: Exploring DMX Network Configurations

The earliest use of DMX512 protocol was for controlling dimmer racks as indicated by the diagram below:
This diagram illustrates the connection of 3 dimmer racks of 96 dimmers each to a single DMX512 universe from a control board.    Each dimmer will take 96 DMX channels for a total of 288 DMX channels.  Since a single universe offers  512 DMX channels there are still 224 DMX channels available on this universe.  Each dimmer rack counts as a single DMX device, therefore, this universe is capable of supporting another 29 devices (up to a total of 32)
Let us suppose that we have 3 additional DMX devices that we wish to control with the console.
The simplest, most straightforward way to communicate DMX instructions to the devices  is to daisy-chin them. This method has the disadvantage of requiring excessive lengths of DMX512 cable to feed signal from the dimmer rack (which is often remote from the stage lighting positions) to the lighting fixtures. However, the 224 extra channels of DMX are available to control these devices.  There is a wide range of DMX devices that could be used here including DMX Irises, Gobo Rotators, Color Scrollers, Automated Yokes, Moving Lights or LED fixtures.  Each type of device (even each model or manufacturer of each type of device) will require a specific number of DMX channels in order to function.  Notice that the diagram appears to indicate that the 3 DMX devices are some manner of moving lights.  According to Brad Schiller on page 20 of The Automated Lighting Programmer’s Handbook: second edition, such a fixture might require anywhere from 16 to 40 channels of DMX.  Assuming the higher number, no more than 5 such fixtures (requiring 200 channels of DMX) would be able to be run in this fashion on the remaining 224 available DMX channels in this universe. (regardless of the fact that this would only account for 8 devices out of the possible 32 allowed).

Another strategy for feeding signal to the DMX devices is to insert a DMX splitter near the board.  This does introduce an additional level of complexity, but the splitter’s optical isolation protects each segment (in the example: both segments) of the DMX512 signal path from electrical faults on the other branch(es).  Since the control board is normally visually coupled with the auditorium and lighting positions, this strategy stands to reduce the length of the cable run from the splitter to the first DMX device verses the previous example, while also making troubleshooting easier.

Alternatively, since most contemporary control boards are capable of out-putting multiple universes of DMX channels, another strategy involves taking advantage of a 2nd universe of DMX in order to control the non-dimmer DMX devices.  This strategy retains the advantages of the preceding method of splitting the first universe, while it eliminates the extra device.  Notice that this strategy would also permit as many as 12 DMX devices each requiring 40 DMX channels on the 2nd universe.

Each DMX device must be assigned a DMX Address in order for the console to address it.  As a rule, the address for each device will be the first DMX channel necessary to address it.  In the example above, the address of the first dimmer rack would probably be 1.  The second dimmer rack would have a DMX address of 97, and the third dimmer rack would have a DMX address of 193.  Meanwhile, in the single-universe examples, the first DMX moving light might have an address of 300, the second an address of 340 and the third an address of 380.  On the other hand, on the two-universe example, the first DMX moving light must have an address above 512.  In this case, it might be assigned an address of 540, with the second addressed as 580 and the third an address of 620.  Notice that in all cases, address numbers for a subsequent device must begin no earlier than the number of DMX channels needed to address the preceding unit. Also notice that I have chosen a multiple of 10 for each DMX fixture address even though it requires the skipping of several potential DMX channels.  This is in the interest of making subsequent programing a little bit easier because DMX channel numbers will be more intuitive.

These addresses are set on and stored inside each device.  There are several ways that these addresses might be entered.  The method of assigning the address for a device depends upon the age, model and manufacturer of the device.  Various methods include:  DIP switches, Rotary Dials, LED display, and digital assignment through software or computer.

That’s it for now! Have fun!  Be safe!

SJM

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