Hi, Ben and thanks.
The problem isn´t DMX-512 reception, the problem is to control a bi-fase steeper motor with PWM (for micro-stepping) while serial line DMX-512 is "interrupting" constantly. I need to capture two channel, first channel will be the position of steeper motor and second channel will determinate the speed of stteper motor (1/2 revolution per minut to 1.5 revolution per second).

Is very difficult for me that all to work correctly this is the reason of my help request.

I think that 50 interrupts per second aren´t many to control a fine micro-stepping for the steeper motor.

I´m sorry not to be able of to write better. I promise to improve (or to try it).