HP48SX - shift keys won't stay shifted - preliminary report

[Bruce Larrabee]

Okay After dismantling the calculator I can report that this problem is not any of the following; keypad, keypad connection to motherboard or the display. I will keep you folks apprised of anything else I discover. Presumably on the motherboard.

[W_Max]

Hmmm... Can you simulate keypresses without keyboard connected (manually short ribbon contact pads)? I'm still sure, that the problem is hardware related. Another guess - faulty power controller - I discover significant voltage drops on some machines, while you press some keys.

[Bruce Larrabee]

Will make both of those checks. Good suggestion.

[Han]

(03-13-2014 03:29 PM)W_Max Wrote:  Hmmm... Can you simulate keypresses without keyboard connected (manually short ribbon contact pads)? I'm still sure, that the problem is hardware related. Another guess - faulty power controller - I discover significant voltage drops on some machines, while you press some keys.

I very well could be misunderstanding something, but would voltage drops only affect whether a key is detected? It seems the issue is not that the calculator never detects a key. It was already stated (in a different thread) that the key presses actually produced the relevant annunciators -- but that it only remained active very briefly.

Is it possible to press (and hold) a shift key to keep the annunciator on? And if so, would it then be possible to do a shifted action by press-and-hold-shift while then pressing a secondary key? I ask because the key handler in the ROM behaves in a specific way. When a key press is detected, it gets saved in a key buffer (including the shifted state). The detection process compares a old keyboard state (i.e. the state of all keys) with the current keyboard state to detect what had changed, and converts this to a key code. The fact that the annunciator turns on and then off suggests to me that something is causing a "secondary" key press (even if you physically only press the key once).

I do agree that the problem is likely hardware related.

[HP67]

What about the possibility of bad RAM? Has the OP run all the self-tests?

[W_Max]

I very well could be misunderstanding something, but would voltage drops only affect whether a key is detected?

I think, the reason is key press itself (some sort of leakage) + activated subroutine (rising CPU current). Other symptom is notable screen contrast cyclic reduce, while key pressed. If power regulator is faulty - drops may be critical for stable CPU functioning, causing "strange" side effects.

[Bruce Larrabee]

I will run these checks as soon as possible (am swamped).

Code:



Han, as to your question, no, to both questions. It is not possible to hold the key in order to keep the annunciator displayed. And thus it is not possible to hold the shift key (any of them and them) and then simultaneously click a key to produce a shifted
key function.