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). A trivial example. The wiring of the serial port on the Epson HX20. It uses an 8 pin DIN plug. As is well-knonw (>) th 8 pin socket will also take 3, 5 and 7 pin plugs). And all of them give a useful subset of the signals : 3 pin : Ground, TxD, RxD 5 pin : adds RTS and CTA 7 pin : adds DSR and DTR 8 pin : adds CD And the serial port on the Tatung Einstein. It uses the quincuncial 5 pin DIN plug, the one that fits either way up. Turning it over swaps TxD and Rxd, and also RTS and CTS. Oh, and a big raspberry to Acorn for the way they wired the same type of connector on the Beeb. Turning that one over swaps the data nad handshake lines, which is rarely, if ever, helpful Another beautiful design is the I2S Model 70 (and 75) Image processor/display systems. I must dig out the manuals and post the details again. I've already mentioned the PERQ (great CPU!) and the HP9100 (the most elegant piece of electronics I've ever worked on). -tony