DMX 512
DMX 512 is now the industry standard protocol used to control intelligent lighting equipment and dimmer packs. It was developed to create a common interface between a control desk and a fixture. The control data is transmitted in serial format using a 2-pair shielded cable, with a transmission speed of approximately 250 Kbps. All the data for the individual 512 DMX channels is sent as a packet, beginning with channel 1 up to the last channel required by the fixture. (Not all 512 channels need used per packet it is very uncommon to find all 512 channels used). As soon as one packet is finished, another one can begin. If nothing has changing the same data will be sent out over and over again.
Each DMX channel contains a level represented by an 8-bit word. This 8-bit word gives 256 individual binary codes for each fixture. (0 = channel off and 255 = channel full on). Some fixture, particular moving lights, will use 2 channels to provide a higher 16-bit resolution.
A typical DMX 512 control desk has the capacity to drive up to 32 units, if you need to drive more than 32 units, or the signal path needs to be split, always use an active DMX splitter. The last unit must always be terminated with a 120 ohm 0.5W resistor across pins 2 & 3. This terminator absorbs signal power which would otherwise be reflected back up the cable and degrade the data.
Every fixture has the ability to select the starting channel number that it will respond to. This is normally in the form of a DIL switch or part of the fixtures digital display. A moving head lamp fixture might require 16-channels to operate, the first fixture would be set to channel 1 while the second fixture would be set to start at channel 17. It is worth pointing out at this stage, when connecting fixtures with different numbers of DMX channels. The fitting with the largest number of DMX channels is first on the channel assignment (1 -16 in our example) then the next device will start at 17 and then 33 etc.
DMX 512 connections are traditional made with 3-Pin XLR style connectors these are a socket (female) on the output and plug (male) on the input. DMX 512 is a transfer of data so standard 2-core microphone cable is not recommended. The wiring used should be a twisted-pair, shielded, low-capacitance data cable designed for RS-485. For any fixtures which use 5-Pin XLR connectors refer to the user manual for the product.
ESR Electronic Components Ltd Cullercoats Tyne and Wear NE30 4PQ