cheesestraws
Well-known member
All in all, this is pretty darn good!
I don't know how I missed all this but this is great stuff! Nicely done!
All in all, this is pretty darn good!
Maybe. The low voltage is more like a symptom rather than the root problem though... I can probably only draw a few milliamps from self-powering, no matter what the voltage. If I can't make it work this way, I may try to add a 3V coin cell battery and run everything off that.Would a voltage boost circuit be feasible?
Is the current draw discontinuous - would a fat cap to draw a constant current, smoothing the spikes in demand, help?Maybe. The low voltage is more like a symptom rather than the root problem though... I can probably only draw a few milliamps from self-powering, no matter what the voltage. If I can't make it work this way, I may try to add a 3V coin cell battery and run everything off that.
Worth checking the specs on the PSU - low currents are difficult to measure and so while the indicated precision is 0.01A, the accuracy at low currents may be worse. It might not be able to (making up numbers) measure currents below 0.05. I remember this being the case with... Either my multimeter of my bench PSU.The bench supply that I'm using says it's putting out 0 amps, to a resolution of 0.01A. So it's something less than 10 mA.
One of my nubus video cards shows the bottom most row of pixels at the top of my monitor. I assume the sync timing is poor.This seems like a scaling bug in the monitor, probably exposed by something non-standard in my sync timing. I think it may be due to the vsync pulse being longer than expected.
Or grounding it / coupling it?I get vertical jittering. With the 5V external supply connected, even if the supply's output is switched off, the picture is stable. If I then disconnect the external supply, the picture remains stable until the next power cycle. If the external supply is completely turned off, the image is still jittery.
I have to conclude that my supply is actually putting out a small amount of current even when its output is switched off, or the output stage has a capacitor in it that's affecting things.
This one looks promising:A charge pump might be an alternative to a boost converter.
What do you mean by no sync? Surely all cards output some kind of sync when active? What I'd pin settings were you using?Nice find on that charge pump IC!
I had a chance to try the self-powered LM1881-based adapter prototype on some computers and NuBus video cards at Mactobefest. The results were not very encouraging, so I stopped collecting data after a few.
Q700 - both composite and h/vsync, 4.79v
"Toby" video card - no sync at all, no video output
Mac II monochrome P/N 630-4385 - composite sync but no video output, 3.2v-3.5v
Mac 8-24 card - no sync at all, no video output, 0 volts
Mac Hi resolution video card P/N 630-4222? - no sync at all, no video output, 4.35v
I did walk away with a Toby card that I can experiment with more.