|HP-42s -- The INPUT Project!|
Message #1 Posted by Paul Brogger on 6 Sept 2001, 8:53 p.m.
With the successful conversion of my HP-42s calculators to 32K, some sort of input/download capability is more desirable than ever. Interested parties may wish to consider the following:
I think a very workable solution would be use PC software and a hardware interface to simulate keystrokes on the HP-42s. This could be done by building a "key pusher" (some sort of device which would push the keys by computer command), or by intelligently shorting the keyboard traces in computer-controlled sequence -- perhaps using magnetic relays.
I like the second approach. If we could bring the keyboard electrical connections outside the calculator (without damaging it, of course), then we could simulate keystrokes by activating correctly connected relays. Relays will easily work at normal keyboard entry speeds, and there should be no concern about polarity or voltage levels that there might be with silicon switches.
If you look at my pictures posted as part of the HP-42s article in the MoHPC, you'll see that the keyboard (http://www.hpmuseum.org/42si2.jpg) is connected to the PCB (http://www.hpmuseum.org/42si3.jpg) through 15 contacts. 15 electrical traces on an exposed mylar strip from the keyboard are pressed against 15 gold contact pads on the PCB when the PCB is attached to the front of the calculator. (This is how the LCD is connected to the PCB as well, but that involves MANY more points of contact.) For the keyboard, there are 7 "row" connections, 6 "column" connections, and two more connections devoted to the ON/reset key.
One might think it necessary to remove the PCB and insert some sort of 15-conductor "ribbon cable" between the keyboard and PCB to access the keyboard traces. Perhaps this would indeed be a viable approach.
However, all 15 of these traces "feed through" the PCB to the other (component) side, where they are exposed without separating the PCB from the keyboard. Therefore, a probably better method might be to solder 15 wires to the PCB at the spots where the keyboard traces feed through the PCB. (This would get around PCB removal and avoid disassembly/reassembly problems.)
I think a tiny 15-pin connector of some sort could be attached to a hole in one side of the HP-42s back case, and the pins carried to the keyboard traces by either of the means just mentioned.
Externally, something like 14 magnetic relays, appropriately wired, could then simulate all the keystrokes possible. If those relays were driven by a PC card interface (or, perhaps, an HP-48GX card?), then programs (each a series of keystrokes) from the controlling machine could be entered into the HP-42s via automated means. These would go in at a relatively slow speed, but would not require laborious manual entry.
The only risks would be: possibly damaging the HP-42s CPU when soldering the connector leads, and when hooking and unhooking the relays. (Perhaps the edge connector could be so designed and connected as to minimize the likelihood of connect-related damage?)
I'm NOT an electrical engineer, but I believe this is a very realistic plan for implementing input into the HP-42s.
Would anyone like to review the feasibility of this approach? I do have two HP-42s units which I would be glad to use in a test of this approach. If the relays could be mocked up and hooked to an RS-232 connector, I'd gladly wire an HP-42s with a corresponding connector and make it available for tests. I'd certainly like to help flesh out the details of the relay wiring, etc. (I just worked out the internal keyboard connection map while diagnosing a problem machine.)
Indeed, there is no need to risk an HP-42s during early testing. Any other Pioneer calculator could be so wired, and its keyboard behavior simulated, in order to evaluate the feasibility of this idea. I've got, for instance, an older-technology HP-14B which should work nicely. An HP-17BII is another good candidate. (And the HP-17B has the advantage that it's very replaceable!) In fact, I think I'm going to go ahead and wire one, and evaluate its behavior using mechanical pushbuttons.
What I need to actually implement a (hopefully, shared) solution is someone expert enough to do the PC (or HP-48) interface part of the project. Do you know of anyone? Does anyone want to volunteer?