DMX512 protocol describes exactly the way according to which data is sent.
The basic element of each data transfer protocol is a low or high level of voltage on the line called
a bit. Its duration in the DMX512 is exactly 4 microseconds. The set of such a few bits determines
transmission code. In our case code length is 8 bits or a byte. This allows 256 different
combinations - e.g. levels of brightness.
Because one frame contains full information about the DMX channel, to send all of 512 channels
we need 512 frames. However, the receiver need to recognize when the entire transmission
starts (i.e. which channel is first). That is way the transmission begins with the signal called
BREAK. It have to last at least 88 microseconds. Another important signal is MARK AFTER
BREAK (MAB). Its length was in the original specification allocated to 4 microseconds. But it
quickly became apparent that many of the devices manufactured at that time had big trouble with
catching up to such a short pulse. That is why in 1990 the change of the MAB to 8 microseconds
was introduced. After transferring the MAB a controller is obliged to send a START CODE byte.
The creators of the DMX protocol reserved this byte to the so-called "future applications". It was
planned to mark with it sending of 512 bytes to control dimmers or other receivers. However,
since the beginning it was not exactly clear what it will be those "other" device, it was assumed
that at the moment its value will be zero - and it stayed so. The starting byte must be zero. If not,
the receiver should ignore all the subsequent bytes.
start
stop
stop
data
1
1
0
1
0
0
1
0
44 microseconds - frame
To a byte of data you need to add a few control bits to determine when a byte is starting, and when
it ends. In the DMX512 there is one start bit and two stop bits. In this way we obtain a sequence of
11 bits called a frame.
BREAK
MAB
START
CODE
44 s
µ
88 s min.
µ
8 s
µ
When you send a zero byte controller starts to send bytes with data, starting from the content of
the first channel. The minimum number of channels permitted by the standard is 24 and a
maximum of 512.
DMX512 protocol - useful information
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