HP9845 design article in EDN

Interesting... Why specifically for me, though (other than I have an HP9845B which AFAIK uses the same I/O backplane PCB). A few comments... For those who don't know the machine, the I/O backplane fits at the rear, behind the PSU. It has 4 edge connectors for interface modules which plug into the rear of the machine. The PCB carries the connecotrs, some bus buffer chips and not a lot else. The problems with the origianl design seem to indicate that 'there is no such thing as groudn' )(with thin tracks, 2 points that both claim to be 'ground' woukld not always be at the same voltage), and that the most complciated part of a digital circuit is the wire :-) I am also a little suprised by the comment that logic analysers didn't really exist at that time (not that a logic analyser would have been a lot of help fo this fault). I am wondering how the older HP machines were actually debugged. Having repaired several of them, I find a logic alauyser to be next-to-essential. Even more so if the design is not known to be sound. -tony