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Tube🍂Time

anyway, i've plugged in a Pentium 90. it should be fine at 99MHz. i don't have the right heat sink so i bent up a section of music wire to hold this junk box heat sink in place. will it work?

26 comments
Tube🍂Time

no such luck 🤔 black screen, no numbers on the display on the front.

Tube🍂Time

all right, i'm very suspicious of the soldering job on this clock oscillator. the upper right pin is the oscillator output and it looks like it may not be connected.

Tube🍂Time

boom! that did the trick! except now the floppy drive isn't working so i can't put in the reference disk 🙃

Tube🍂Time

floppy drive issue seems to be this leaky electrolytic capacitor. it was working before, so i guess it must have been slowly degrading.

Tube🍂Time

ugh. i have this same issue with another Type 4 complex I have (486DX2-66). this is probably logic analyzer time now :blobsweats:

Vito Sartori replied to Tube🍂Time

@tubetime logic analyser time is always a good time! 😅 Best of luck, tho! <3

Tube🍂Time replied to Tube🍂Time

here we are again, logic analyzer cables snaking out all over the place.

Tube🍂Time replied to Tube🍂Time

with 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).

Tube🍂Time replied to Tube🍂Time

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.

Tube🍂Time replied to Tube🍂Time

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]

Tube🍂Time replied to Tube🍂Time

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.

Tube🍂Time replied to Tube🍂Time

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).

Tube🍂Time replied to Tube🍂Time

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.

Tube🍂Time replied to Tube🍂Time

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.

Tube🍂Time replied to Tube🍂Time

OH! it's booted off the reference disk!

Tube🍂Time replied to Tube🍂Time

ohh yeah, 90MHz! well, technically, 99MHz (the oscillator is 66MHz instead of 60MHz).

aeva replied to Tube🍂Time

@tubetime shouldn't Operating System/2 Version 3 just be OS/3

Tube🍂Time replied to Tube🍂Time

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.

Tube🍂Time replied to Tube🍂Time

(i've already disconnected all the logic analyzer cables, so i'll have to look at it some other time.)

Tube🍂Time replied to Tube🍂Time

well, that was fun--i'm glad i was able to get it to boot.

Chartreuse replied to Tube🍂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

Darryl Ramm replied to Tube🍂Time

@tubetime Half a warped OS...

The whole IBM fumbling the Star Trek thing with OS/2 and the Warp name silliness was pretty amusing.
e.g. arstechnica.com/information-te

Chartreuse replied to Tube🍂Time

@tubetime I like that as a concept. Simple and elegant without any state machine complexity.

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