So who knows stuff about the #commodore64 light pen?
Was there an amplification circuit for the light sensor input?
It clearly is responding when I point it up at my task light, but it's barely 30mV. Just no idea if that's what I should be seeing. Also no idea why the 'floor' goes negative...
One button is 5V high and properly drops to 0 when pressed as expected. the other button doesn't seem to work, but the CGA only supported one button anyway.
Not an expert but tried to do some sleuthing.
The Vic-II chip datasheet http://cini.classiccmp.org/pdf/Commodore/ds_6567.pdf doesn't say anything special about the light pen input, so I would expect it to be a logic level input. Input high level 2.0 volts minimum.
Does the commodore 64 PCB amplify the signal? https://www.zimmers.net/anonftp/pub/cbm/schematics/computers/c64/250469-rev.A.pdf No, not that I can find. On page 3 in grid E3/4 a series inductor L3 is shown, without a value.
https://www.c64-wiki.com/wiki/Light_pen shows a typical schematic of a light pen. The light sensing device functions as a PNP transistor. (It's shown upside down in that diagram compared to a typical schematic! And the connections are unlabeled. Ugh. 6 is "button1", 7 is +5V and 8 is gnd) When it is DARK, the transistor does not conduct, so the pull up R1 produces a voltage of close to +5V.
When it is illuminated, the light produces a base current and conduction occurs. In this case, the voltage should drop to close to 0V.
Since you are seeing low voltages but not high voltages, check that the pull-up is present and has a plausible value; and that +5V is present.
@stylus thanks for the info, I was just about to try adding a pullup on the strobe line.
to be clear, there is no commodore 64 actually involved here, I just have the pen, and I'm givijg it 5V from my bench supply.
The goal is to see if I can get it working with my IBM CGA card...
gloriouscow
Stylus