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 512.
This means that even if we have a simple 6-channel lam controller, the
transmitter still have to send 24 DMX channels. The interval between each
frame of data is marked as MARK BETWEEN FRAME (MBF) and can range
from 0 to 1 second. Thus, a complete DMX package looks as below:
Obviously, sending time of one DMX data set depends on the number of
channels. For a minimum of 24 channels it is 1196 microseconds, and for a
maximum of 512 channels it is 22668 microseconds. Because of that, the
maximum transmission change rate value is 836Hz (for 24 channels), and in
the second case only 44hZ (for 512 channels). This is quite a serious
limitation, whose effects can be observed during fast movement of e.g. a
large number of scanning mirrors. Another quite important parameter is the
minimum frequency of sending DMX packets. According to the standard the
rate is 1Hz. This means that the interval longer then 1 second is considered as
no DMX signal. Unfortunately, creators of this protocol did not defined what
should do in this case the receiver. There are several possibilities:
•
maintain of the recently received DMX value
•
immediate shutdown of device (BLACKOUT)
•
smooth device shutdown
•
during the lack of DMX it sends a previously predetermined value
21
BREAK
BREAK
MAB
M
B
F
M
B
F
M
B
F
M
B
F
M
B
F
START
CODE
DIMMER
1
DIMMER
2
DIMMER
3
DIMMER
511
DIMMER
512
full DMX packet
