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 BRAKE
(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.
20
start
stop stop
data
1
1
0
1
0
0
1
0
one frame 44 microseconds
BREAK
MAB
START
CODE
44 s
m
88 s min.
m
8 s
m
