Thank you Neil, very informative. I'm not sure how I missed it when first reading the manual !

Hi

How does the battery gauge works on the S4 ? Is it based on battery voltage ?
I'm feeding a supply from a PSU to the battery terminal and varying the voltage doesn't seem to affect it, it always shows "full".

Same for charging, is it purely based on time or does it monitor charging current ...

Hi Neil,

I know the S4 is ancient, actually almost 35y since release, but it goes well with what I want to use it for :)

I was hoping that you had higher resolution of the pictures you posted. It wouldn't be perfect of course since many things are obscured but it would be a start. But as it stands ...

Hi Neil,

Any chance for higher resolution pictures or some theory of operation or schematics ?
Looks like one 74hc595 and 2x 74hc166 and some passives.

I'm in the process of restoring two S4s and programming GAL/PAL for retro computing hobby is also something I was hoping to use them for.

Cheers ...