Another way to test, put an led on the pin, and use a test sketch of 100 pixels that contains white, over and over again as fast you can. You will need to call Dirty just before Show to make sure it sends. The led should show some light. This is under the theory that the led will just see the data train as a PWM light level, as the color white is 66% duty cycle with an unperseptible flash from the latch in the data train .