PD busy is indeed an indication that your processor is busy.
The communication is fairly simple, only 3 data lines.
Depending on the combination , the relevant message ( PD busy ) is displayed.
But this message is NOT displayed if you de-actvated the communication to the processor.
This is done in the "service menu" of the imagesetter.
Did you ever go there, or see somebody else do that ?
The disadvantage of a disabled communciation is an imagesetter error when the plate remains in the unload area.
W