slick bits in computers (WAS: VAXen Rule!)

The keyboard on the HP9810 and HP9820. OK, not really a computer (they're proggammable calculators, but hey, they've got a bit-serial 16-bit processor, I/O (HPIB was avalable for the '20, are user programmable, etc). The PCB under the keyboard has spiral tracks under each key, connected by vias. These forma a pair of pulse transformers, with the primaries and secondaries in series, but in antiphase. Thus a pulse through the peimaries produces no output, since the pulses in the secondaries cancel each other out. Pressing a key brings a disk of metal against one side of the PCB, damping that transformer, and the pulse from the other one then appears on the output. The primaries form an X-Y matrix that's scanned by electronic under the keyboard, the output pulses are detected, and the state of the scan counter is then fed to the processor. Actually, I've missed out one detail, there are 2 keys at each location of the matrix with the secondary pairs connected in antiphase. Pressing one key produces a +ve output pulse, the other key a -ve pulse. These pulses are detected by a LM711 comaparator that looks for one polarity of pulse, then the outher (each location in the matrix is thus driven twice in succession).