9810 question and offer

[Tony Duell]

(02-27-2014 05:33 AM)dramsey Wrote:  I notice the keyboard on the 9810 is one of those wherein pressing a key simply moves a metal disk close to a printed coil on the underlying PCB, presumably registering a key stroke through some sort of inductive coupling. I imagine this makes the keyboard very reliable mechanically, and I also imagine the supporting circuitry is kinda complex.

The keyboard is quite interesting. There are 2 spiral PCB tracks under each key which form a pulse transformer. The keys are connected up in pairs with the 2 transformers in antiphase. So that normally, when a drive pulse is sent through the primary windings (tracks), the induced pulses in the secondaries cancel out.

When a key is pressed, the metal disk damps that transformer, so it produces little output. Therefore the signal from the other transformer gets through and is detected.

The primaries are wired up in matrix, there's a scan counter and decoders to send a pulse through each pair in turn. The secondary pairs are all wired up in series, and connected to the input of a comparator chip that detects the induced pulses.

The same keyboard design is used in the HP9810 and HP9820 (and I assume the HP9821), but AFAIK in nothing else. The HP9830 uses a conventional set of (Cherry) keyswitches.