Gregory's Serverwith the *working* older type 2 complex, floppy drive read operations begin DMA by trying to negotiate arbitration level 2 on the bus (see ARB0, 1, 2, and 3).
|
17 comments
 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] 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. 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). so i can confirm this by disabling the output driver (pin 12 of U37) for ARB1, which will allow it to remain high, so U26 will drive a 2 on the bus instead of a 0. a little soldering work and a small piece of kapton tape, and i think we are ready. and the logic analyzer confirms that, with this mod, we're seeing a 2 on the bus even though U26 is bugging and trying to drive a 0. ohh yeah, 90MHz! well, technically, 99MHz (the oscillator is 66MHz instead of 60MHz).  strangely enough, when i connect my other type 4 complex (a 486DX2-66), i get the same DMA timeout error, so this "fix" is definitely a hack and doesn't really fix the root cause. (i've already disconnected all the logic analyzer cables, so i'll have to look at it some other time.) @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. |
but with the *nonworking* type 4 complex, the floppy read operation begins by trying to negotiate DMA arbitration level 0! huh, the floppy controller is supposed to be hard wired to level 2. this is definitely a problem.