Serial input for an HP42S?



#5

After reading some of the HP42S postings I had a thought:

Can the HP42S detect (i.e. is a flag set) when the battery is low. This could form the basis for an input device by (carefully!) modulating the battery voltage. An HP42S software routeen could then read the 'battery low' flag (perhaps using the i/r output for handshaking) and use this serial input for loading data.

Perhaps this is beyond what the user can program or is there another reason for it not working?


#6

Tom:

Around 1981, I have tried (unsuccessfully) to use the Flag 49 (LoBat) as input means for an HP41C. I used a silicon diode in series with the batteries, with a simple switch in parallel with the diode. Opening the switch lowered the batteries voltage by some 0.65 volt, closing it raises the voltage to its normal level again. However, I was unable to reliably cause the flag to respond to such "modulation". Even if I managed to set the flag, then it will only reset when turning the calculator off and on again. While OFF helped with the first part, and the AUTORUN Flag 11 helped starting my program at power on, the lack of a timer means I must be there to press the ON key each time. (BTW, we have no timer on the 42S, indeed).

I am not sure about the 42S case (it may behave a little differently, and its batteries add up to 4.2 V, instead of 6 V), but it may prove very difficult to achieve any decent serial operation in this manner.

In the memories forum, here at the Museum, you may find my article where I comment these expreiences and the more successful tests on using the TONE instruction to deliver square-wave pulse sequences that could work as a OUTPUT signal from my HP41C.

Just as a possibility, would you think about wiring some kind of CMOS analog switches to the keyboard, and so produce "keypresses" from an external device? It should be not very difficult, and GETKEY is a rich instruction, even with timeout detection...

I will not risk my mint HP42S on such experiments...

#7

I think this is impossible for at least 3 reasons.
1) There is enough capacitance across the supply lines in the 42S (the capacitor that maintains memory when you change the batteries, for example) that for any sensible PSU you'll only be able to modulate the supply rails very slowly, so you'd have a very low data rate
2) If the low battery flag is anything like the one on the 41, it's not a simple output of the low battery detector. It's set if the machine detects low batteries and is only cleared when the machine is turned off and on again. Not a particularly practical way to transfer data.
3) AFAIK a user-language program on the 42S can't modify (or write) another user-language program. So even if you could download bits this way, you couldn't store them in program memory. And what is
wanted, of course, is a way to load programs into the 42S

#8

I agree with Tony's assessment. Elsewhere in the forum (under "I/O for non-I/O-enabled calculators") you'll see a discussion of various ways to get in via the keyboard.

I think this CAN be made to work.

If someone can help with the PC interface, and we can agree upon a connector, I'll wire one of mine for testing.


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