I once was tasked with updating some very expensive equipment to add functionallity and speed. The thingy ran on (was it 4 or) 5 ~ 14" by 14* boards on a 100 pin bus. The task was to replace these cards and drive the machine from a PC. The lawyers told me that I could not even glance at the schematics of the machine. All I could do was to operate it with the knowledge an operator would have, but no more. I designed a 14 * 14 card with (was it 10 or) 12 of the then available CPLDs from Intel (forgot the number, but it was basically 8 22V10s in one package). I then programmed the card to function as a logic analyzer and started analyzing the signals for the various operations. Then I reconfigured another identical card as a machine driver pulled the 5 cards, inserted the driver and went to work. A number of times I had to resinsert the original cards and my 'analyzer' to refine my knowledge of a detail but, in the end out went the 5 cards and in went my card and a PC.

Erik