First post, by jsp
Hi!
Some years ago I damaged one of my 486 boards (a PCChips M912) while trying to remove the BIOS ROM chip to replace it with an updated version that would allow me to use more modern CPUs. While I managed to patch my way around it, the board works intermittently due to the unreliable connection. The main issue is that some of the plated holes are gone, so a proper fix is not trivial to do.
Hence, I am trying to find alternatives to bring this board back to life. I started testing for continuity of the pins of the BIOS connector in the board, and most pins are tied (or so it seems) directly to the ISA bus. In particular, the lower 16 bits of the address bus (which makes sense) and the 8 lower bits of the data bus. There are two extra pins (Chip Select and Output Enable) that are only present in the BIOS connector. My idea was to use an ISA card making the connections between the ISA bus and the corresponding pins of an eprom chip. The CS and OE pins would need to be wired directly from the original BIOS connector to the board.
The main issue that I am finding is that despite my multimeter showing continuity between the data bus lines in the BIOS connector and the ISA Bus, the documentation that I've found for some chipsets seems to imply that the data bus for the ISA bus and the BIOS chip are not directly connected, the former being SD[0:7] and the latter 🤣[0:7].
In my own experimentation with a custom made ISA board, the motherboard starts booting up and showing some POST codes, but eventually freezes with code 06 (after going through a bunch of other codes).
Any ideas of what could be going on? Would be great to have the schematics for this board to confirm if the data lines for the BIOS connector are 🤣 instead of SD.
Thanks in advance for any ideas!
