With MAC 600 main CPU firmware version 2.4 and earlier, the shutter continues to open and close if a strobe value is set on DMX channel 1 and the dimmer is closed (DMX 0 on channel 2). Due to a small amount of light leakage through the dimmer wheel, there will be a visible flickering if one is looking directly at the fixture. To avoid this, do not use the shutter strobe function when the dimmer is closed.