DMX512

The original standard was developed and published by USITT in 1990. It is now an ESTA publication, and goes under the title of ''Entertainment Technology - USITT DMX512-A - Asynchronous Serial Digital Data Transmission Standard for Controlling Lighting Equipment and Accessories''. It is also known as E1.11 or DMX512-A. The standard may be purchased in PDF from the ANSI Electronic Store.

DMX is an RS485 (differential) signal, that has + and - signal wires as well as a common reference wire. Some will suggest that DMX (and therefore RS485) is a two wire system and that the common reference ("ground") is not required. This is a myth and should not be perpetuated. [more detail to be added later]

It is asynchronous data with a baud rate of 250,000 bits per second. There are 513 bytes per DMX frame (packet of data). Each of those 513 bytes consists of a start bit, 8 data bits and a stop bit. At the end of each 513 byte block, a special sequence of signal levels indicates the end of frame. This allows DMX receivers to synchronise correctly with the incoming DMX stream.

The first byte is called the start code. For most applications, this byte is always zero. When DMX receivers (dimmers / controllers) see a non-zero start code, they should ignore that DMX frame unless they use that data for configuration purposes.

The following 512 bytes are the channel levels. Each one represents 256 different levels, from 0 (fully off) to 255 (fully on). Some equipment may use pairs of channels as high/low bytes for 16 bit (65536) level data. This only works however if the sending equipment utilises the same pairing.

The original official connector for DMX was the 5 pin XLR. Due to the expense of 5 pin XLR plugs and sockets, some (mainly Chinese) equipment manufacturers use 3 pin XLRs instead. Over the years, this has led to people assuming that balanced microphone cables with 3 pin XLRs are a suitable substitute for DMX cables.

While short microphone cables may work in some cases, they do not have the correct electrical characteristics for successful transmission of high speed DMX data. Everyone has a story to tell about how their DMX system works fine with mic cables. In real life however, the results are unpredictable and not worth the hassle. Using the correct cable type (example) is simply a smart choice and will pay off.

In more recent times, the standard (DMX512A Standard E111-2004) has been updated to include the use of RJ45 (network) connectors and CAT5 cable. The standard recommends that it is only used for fixed / infrastructure wiring, as the connectors and cable are generally not designed for frequent connection and movement. Some stage lighting professionals use a toughened version of CAT5 cable in touring applications.

DMX uses a "daisy chain" topology (layout). This means that the cable chain starts at one end (usually the DMX transmitter), looping in and out of each DMX receiver. Unless you use an active splitter, you '''must''' wire your DMX receivers (dimmers / controllers) in this daisy chain style.



Tapping off the DMX chain with long leads is strictly forbidden as it violates the RS485 spec by introducing stubs (shown going to the lower dimmers in the diagram below) of an unacceptable length to the topology. The high speed DMX signal requires termination at the far end, but when long stubs are introduced, this becomes impossible as there are now multiple far ends. In reality, most DMX devices have a short stub internally going from the DMX connectors to the circuit board. Stubs like these are typically under 150mm (6") and do not have a significant impact on the data integrity.



Using Y leads to generate a star cable layout is also forbidden, as it results in more than two ends in the DMX chain. It may sound tempting, and it may make your wiring easier, but it will introduce errors in the DMX data stream. These errors may show up as intermittent flickering or seemingly unresponsive channels.



If you do use an active splitter (see below), each splitter output is the start of a new chain, so you can implement your star wiring configuration correctly.

The RS485 standard specifies the maximum number of nodes (dimmers, controller, etc) you can have in a single DMX chain. This is specified as 32 "load units". Traditionally, one RS485 receiver (sometimes referred to as a DMX chip) was equivalent to one "load unit". This meant that you could have 31 receivers in the chain, as the transmitter also counts as a unit.

Newer variants of the venerable MAX485 IC have a load rating of 1/4 or 1/8. This means that you can use 4 or 8 times as many of them on a single line. In practice it is best to assume that each device still represents one load unit, as there is no easy way of knowing without checking the IC type and looking up its data sheet.

Please note that this node limit applies to each single DMX chain. On an active splitter, each output is independent and capable of driving another 31 load units. ( the splitter counts as one )

A DMX signal chain must be terminated at the far end. This is accomplished by either setting a jumper, or plugging a terminator (plug with 120 Ohm resistor) into the end unit. The DMX signal chain must not be terminated at any place other than the end. For DMX receivers with on-board termination jumpers, this means ensuring the jumper is off at all intermediate units.

The RS485 spec (that DMX is based on) does specify that termination be applied to both ends of the data cable. In reality, almost all of the time one end is the DMX transmitter, and this negates the need for a terminator at the "start" of the DMX chain.

Failing to terminate your DMX chain correctly can have various results. It may work just fine, you may see flickering channels or it simply may not work at all. Correct termination is one of the first things to check when intermittent operation is observed.

For DMX receivers that have separate DMX in and through XLR connectors, you can make a terminator by soldering a 120 Ohm 0.25W resistor between pins 2 & 3 of a male XLR plug. This plugs into the "through" or "out" (female) XLR connector on the last DMX unit.

The only method of using a "star" topology with DMX is via the use of active splitters. These accept a DMX signal and regenerate several new DMX outputs, each one the start of a new chain. DMX splitters can also be cascaded if required. Electrical isolation is not a requirement of DMX splitters, but it is a good idea, as it can limit damage to a small area if nasty things happen to one of the DMX lines. At the very least, a DMX splitter should employ isolation between its input and outputs to protect the lighting desk or PC.

• DMX Adapter Leads For converting between XLR and RJ45 plugs in a variety of pinout configurations for ESTA (DMX), LOR and Renard.

• Guidelines for Proper Wiring of an RS-485 (TIA/EIA-485-A) Network] by Maxim http://www.maxim-ic.com/app-notes/index.mvp/id/763
• Recommended Practice for DMX-512-A] by Adam Bennette (Google Books) (plasa)
• RS485 cables - why you need 3 wires for "two wire" RS48 http://www.chipkin.com/articles/rs485-cables-why-you-need-3-wires-for-2-two-wire-rs485 5