Home » worth knowing » DMX – Digital Multiplex

DMX – Digital Multiplex

July 8th, 2013 Posted in worth knowing Tags:

70064550d DMX Commander

How digital controlling protocol works!

Every retailer is familiar with it, the standardized process to digitally send controlling information to dimmers, „intelligent“ spots or effect devices – DMX: But how exactly does it work and whose idea was it?

Digital data transmission has been around since the 80s. But controlling consolemanufacturers used differing systems, incompatible with each other, so the customer was forced to buy spots and controllers from the same producer. This changed in 1986 when the DMX standard was introduced, based on an initiative of the USITT (United States Institute for Theatre Technology). After some improvements were made and problems were solved, it was realized four years later. The common and still valid name of the standard is DMX-512 (1990). Even though some work has been done on it since then, there’s currently no newer version.

DMX Timing chart

To understand how it works, let’s have a look at its name and number: 512. This represents the number of channels which can be used for data transmission, not to be mistaken with the number of devices that can be connected to the DMX-512 controller. Since “intelligent” lighting effects are being programmed with more and more functions, this calculation doesn’t work out. For every single parameter, as for example a gobo wheel or a mirror, one channel is assigned. For a scanner, the calculation goes like this: There are 2 channels for PAN, 2 channels for TILT, 1 apiece for brightness, strobe, first gobo wheel, second gobo wheel, gobo rotation, first color wheel, second color wheel, effect wheel, focus, and zoom. That makes 15 channels for one device. Due to the particular channel allocation and the maximum amount of channels available, 34devices at most can be connected to one DMX controller having no additional splitters.

Information is sent as a constantly repeating data package. It consists of starter bits of data, which informs the receiver about the fact that the package has been refreshed. Then, the package sends a serial data stream (250 kbits/s), which gives information about the size of the channel. Theoretically, this all happens from channel 1 up to channel 512. The channels are separated from each other by special start and stop data.

Metaphorically speaking, the whole structure can be compared to the postal system. Each post man has a district of 512 houses. Each house has its own address. Some houses are bigger and comprise several apartements – then you have several channels for one device, like for the scanner mentioned above. The post man now goes along from house to house and delivers the mail – the data – into the mail boxes. Each recipient now opens his own mail box and takes the mail out. At the same time, each recipient is informed about his address – one of 512 – and ignores the mail (data) that has not been sent directly to him. „Intelligent“ spots for example have one address as a starting point and receive data for this address or for sub-addresses. This is comparable to a building, where the mail is gathered at the reception and then distributed to the people living in the house.

This address determination makes controlling extremely easy, as every device and parameter has been exactly defined. Important for the addresses, however, is the existence of a load resistor called terminator. This terminator eliminates interferences by signal reflections. Normally, this task is taken over by the last device in the chain.The terminator is plugged into the DMX output of the last fixture with a 120 Ω resistor between Signal (–) and Signal (+).

Meanwhile, there’s also wireless DMX via radio transmission. This technology is a further step to eliminating problems, as it substitutes the seemingly endless cables by invisible radio waves, which will certainly not cause you to trip. For additional hardware, you only need a sending and a receiving unit.

2 Responses to “DMX – Digital Multiplex”

  1. @ Thanks for your Post; it really provides me lot information regarding my work.

Leave a Reply