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Top-level
 the floppy controller chip (upper left) doesn't natively support MCA bus, so the chip in the middle (U26) implements that for it. Specifically, U26 uses an external open-drain buffer (U37, bottom) to drive the ARB lines. but it *senses* those same ARB lines with 4 separate pins. [most likely U26 is a semicustom gate array with a limited number of possible open-drain outputs] 15 comments
my theory is that for some reason, U26 (85F0464) isn't working correctly and is driving 0 onto the arbitration bus when it means to drive 2. so it thinks that it has lost arbitration, but it actually won it. then it hangs in a halfway state, and the DMA controller throws the timeout error (presumably by asserting NMI). @tubetime always felt weird that even on IBM hardware that supports it, that OS/2 cant do an ACPI soft shutdown. Even OS/2 4.52  @tubetime Half a warped OS... The whole IBM fumbling the Star Trek thing with OS/2 and the Warp name silliness was pretty amusing. @tubetime I like that as a concept. Simple and elegant without any state machine complexity. |
Gregory's Server
the complexity comes from the arbitration logic. it's like an open-drain priority bus, so the lowest binary value on the bus "wins" and the expected output matches the value on the bus.