Cross-talk square-wave?

[David Bridgham]

And I think this picture is the smoking gun.

http://pdp10.froghouse.org/qsic/pic_24_2.gif

Again, the bottom trace is the CS signal in question and the upper trace
is now one of the QBUS DAL lines (after the bus transceiver and level
converter) that's running across the ribbon cable near the CS signal. 
It does appear that induction can make a fairly clean square wave.

Well, the purpose of a prototype is to learn and this has been one
learning experience after another.