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.
The cable:
The
cable 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 (0v)
Pin 2: -S
Pin 3: +S
-S and +S are normally somewhere between +12v and -7v.
Signal:
High (1): +S > -S
Low (0): +S < -S
(The difference between -S and +S must be at least 0.2v for a
signal to
register, 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.
A packet:
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.
A frame:
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'