There are control lines on the ISA bus to differentiate between I/O port reads / writes and memory reads / writes. There was activity on the I/O pins, they lead to a 244 I already replaced. The memory write line however never sees any activity. And since we know this board gets very far in the boot process, the video BIOS gets passed control to clear the video memory. Which never happens however. So I followed that MEMW control line to ..
And who would have thought. This gate is dead.
Yellow is out, blue in, red output enable (active low).
Yellow is out, blue in, red output enable (active low).
What I need now is a small compact flash card with all the DOS hardware testing tools. If you have suggestions beyond the usual CheckIt and Norton Sysinfo, please post them in the comments. I will share the image file once I'm done
Tbh I can't quite believe I managed to bring it back to life. I thought the chipset was toast. Luckily it has all those buffers on there. I think I will pull the other suspect buffers too, lest they put a strain on the chipset due to bad input gates.
Bun's Lab
I've already replaced a bunch of 74F244 buffers and 74F245 bi-directional bus transcievers because they had wonky looking outputs or even inputs
Actually, I just found those ICs that I already pulled back when. At least one actually tests bad in the minipro.
Remember kids, ESD is real. Don't give your boards carpet time. Ever.
If you think it's fine, you are wrong. Eventually it will start to glitch and act weird. Good luck then replacing ASICs and hybrids.
If you think it's fine, you are wrong. Eventually it will start to glitch and act weird. Good luck then replacing ASICs and hybrids.