DMX512, is a control protocol, used in most theatres to control dimmers and intelligent fixtures. DMX, is the replacement of AMX192 which is an analogue system that is based around using a voltage difference between 0-10V to control lights. DMX 512, has many advantages over the old AMX system: DMX, can be used to control intelligent moving lights, and is much easier to interface with computer based systems.
The DMX system consists of what are referred to as channels, and the value of each channel varies from 0 to 255. However in the case of a dimmer, this would normally be changed on the controlling device so that the brightness was displayed in a percentage form. Each XLR cable run, can carry up to 512 channels, hence DMX is referred to as DMX512.
DMX 512, works on a system of highs and lows, and packets of data; these packets that tell the fixture or dimmer what to do and are refreshed as soon as the last packet has finished being interpreted. When there are no channels transmitting data in what is known as a 'no-data' situation, the DMX control device will output a continuous high signal. At the start of the next DMX packet, there is a Break, and this is then followed by the Mark After Break, this followed in turn by the Start Code and this again followed by the Mark Time Between Frames which is followed by the Channel Data, which is finally followed by the Mark Time Between Packets.
DMX can be transferred through various cable types, the three most common, are XLR5, XLR3 and RJ45 Ethernet. Originally the standard was XLR5, as whilst DMX only utilised 3 wires, there were two extra cores, to allow for possible expansion, and also separate DMX cable, from microphone cable. However more recently, many manufacturers have started using XLR3 connectors as it allows the utilisation of microphone cable, which is often more easily available. RJ45 ethernet connectors began to be used, as it allowed the networking of all DMX devices, as both lighting desks, dimmers and some newer intelligent fixtures began to have networking capability, a feature which could be found extremely useful in larger theatres.
More information regarding DMX, can be found on the U.S. Institute of Theatre Technology's website, as DMX512 is based on their standards