Check out this circuit that looks promising:
This is a simple O'scope trigger circuit for checking DMX signals. It is based on a 4528 CMOS monostable multivibrator.
The "trigger" pot is adjusted first, untill a short pulse is generated near the end of the "BREAK" portion of the DMX signal. Then adjust the "delay" pot to zoom in on a portion of the signal. On DMX signals with few channels it works to trig the scope on the "trig" output, rather than the "delay" output.
This circuit will not work well on DMX signals where the "inter frame time" is as long or longer than the "break". This is a legal condition, but doesn't show up to often.
It is best to keep this circuit "floating" by powering it from a 9 volt battery and a 78L05 voltage regulator(not shown in schematic)."