DMX-512 is the standard by which the majority of theatre
and concert lighting
, smoke machines
, and other dynamic
items are controlled
. Up to 512 channels
can be run on a single line.
itself is either a 5-pin, or a 3-pin cable. 5-pin cables seem to be more popular, although the extra 2 pins are redundant
. In both the 3-pin and 5-pin cables, the pins are laid out as follows:
Pin 1: Ground
Pin 2: -S
Pin 3: +S
-S and +S are normally somewhere between +12v and -7v.
(1): +S > -S
(0): +S < -S
(The difference between -S and +S must be at least 0.2v for a signal
, although, with a range
of +12v to -7v, this shouldn't be a problem.
Pulses are sent at a frequency
of 250 khZ - that is, one pulse every 4us (microsecond
The signal is broken up into packets
. These contain a number of frames - each frame containing the information for one channel
In a packet, the DMX controller
will send out a frame for every channel up to and including the highest channel
number that has changed since the last packet
was sent out. This means that the frames do not actually need to have information on the channel
number - the DMX recievers
just count up until it gets to a channel
number that it recognises.
So, if the only channel level that was changed since the last packet was channel number 15, then the DMX controller
will send out the current data for all of channels 0 to 15.
Before a packet is sent out, the DMX controller may be sending out a constant stream of 1's, in an idle
state. To tell the recievers
that there is some data about to go, there is a BREAK signal
, which is at least 22 pulses of 0 (88us). This is followed by a Mark After Break (MAB) which is at least two pulses of 1 (4us). This is then followed by a number of frames.
Each frame will start with a single
START pulse, which will be 0. This is then followed by a series
of 8 pulses, which is the DATA. This is the level that the current channel
should go to, and can be, in decimal
, anywhere between 0 and 255. At the end of the frame are two END pulses, which are both 1.
The first frame in each packet
(frame 0) will always have a data
value of 00000000. This signifies that the following information is for dimmers
. At the moment, this is all it can be, but in the future
, it could specify a different kind of reciever
After all the frames are sent, a new packet can begin with a BREAK and MAB, but often a Mark Time Between Packets (MTBP) is inserted, which is a series of pulses of 1 (as if there was a slight idle
between the packets
If you're after any other theatre tech information, then do take a look at 'Everything you ever wanted to know about theatre tech, but were afraid to ask