Being a nuts and bolts engineer, my technical knowledge of avionics is limited to finding the big knob labelled ON-OFF and suchlike ... however, I found a useful article from Avionics Magazine, "Feedback over PEDs", April 2000, pp49. I have it scanned to archive if you don't have access to a copy elsewhere conveniently and can send it via email.... hopefully that isn't a dreadful contravention of copyright ...