Deunan wrote on 2021-05-02, 18:55:

So let me ask about IMD, because I see some clarification above about it working now? Or did I misunderstand? If IMD does recognize disk format when you enter the alignment option, and more importantly can read the sectors without timing out or freezing when you press D, and it's only DOS commands that fail on the floppy drive - then the issue might not be a HW problem at all. Because IMD does it's own low-level stuff, bypassing BIOS, it takes over the interrupt handler and programs the DMA channel 2 directly.

As far as I know, IMD only checks by reading the FDC registers, not reading the actual data of the disk. So that could work without DMA working. I think. I've learnt a little recently about floppy drives, but don't know much about the detail the FDC upward. Apparently the first set of weird numbers were due to an unformatted disk, so I think what it's showing is that the FDC is correctly reading the disk, showing the correct track and sector numbers, and not getting any CRC errors.

The only good difference between working system and non-working system so far has been this DMA issue, so I think that's worth chasing a bit further. If the '461 pinout is similar to the '471 then the '151 DRQ multiplexor is shared with De-Turbo, WIRQ, RC and IOCHCK. They could be the cause of Z switching (I think there's a mix of active high and low signals in there), which i can see makes working out if DRQ2 is appearing on the output a bit difficult.

Looks like SN74F151 are available: https://www.mouser.co.uk/_/?Keyword=74f151&FS=True

The Z̅ signal might not route anywhere. The '471 datasheet just uses the non-inverted output.

I really dislike desoldering stuff from mobos, because sometimes the old traces/vias get damaged and there's even more stuff to fix, but with no other tools and ideas I would try to desolder this '151, and the other one that has IRQ12 connected to it. Put sockets on mobo and swap those two. If the DRQ2 input is busted then you'd loose IRQ12 instead by swapping the chips, but that IRQ is not used much so it's a preferable quick fix. And then you can start the hunt for a 74F151 (or maybe more modern 74ACT).

I like the sound of that, although it's not my board at risk. Are there any DOS tools for displaying DMA activity? I'm wondering if it's possible to check in software if the chipset is asserting DACK2?

maybe that particulare '151 is faulty after all. I really dislike desoldering stuff from mobos, because sometimes the old traces/vias get damaged and there's even more stuff to fix, but with no other tools and ideas I would try to desolder this '151, and the other one that has IRQ12 connected to it.

This kind of solder work is my underpaid full-time job, so 8-pin DIP's crammed between plastic edge connectors are a piece of cake. As it turns out, our suspect chip had pin 2 fall off.

Sure enough, switched chips around and the floppy came back.....and we lost the IDE controller. :V
Switching the chips back reverts this so the 151 is certainly the fault. I'll get a new one ordered tonight but hurrah that was a tough one.

As for the funky imd tracks and stuff I'm putting that down to a corrupt floppy that needed a format. IMD does specifically ask for you to insert a formatted disk in for the test.

Edited: Oh, nope I guess the leg fell off again. Resoldered it and both floppy and IDE are working again. Currently testing for stability but I think we are good.

CelGen wrote on 2021-05-02, 22:58:

This kind of solder work is my underpaid full-time job, so 8-pin DIP's crammed between plastic edge connectors are a piece of ca […]
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maybe that particulare '151 is faulty after all. I really dislike desoldering stuff from mobos, because sometimes the old traces/vias get damaged and there's even more stuff to fix, but with no other tools and ideas I would try to desolder this '151, and the other one that has IRQ12 connected to it.

This kind of solder work is my underpaid full-time job, so 8-pin DIP's crammed between plastic edge connectors are a piece of cake. As it turns out, our suspect chip had pin 2 fall off.

Sure enough, switched chips around and the floppy came back.....and we lost the IDE controller. :V
Switching the chips back reverts this so the 151 is certainly the fault. I'll get a new one ordered tonight but hurrah that was a tough one.

As for the funky imd tracks and stuff I'm putting that down to a corrupt floppy that needed a format. IMD does specifically ask for you to insert a formatted disk in for the test.

Edited: Oh, nope I guess the leg fell off again. Resoldered it and both floppy and IDE are working again. Currently testing for stability but I think we are good.

Great idea and great work !! I did not suspect a blown gate on TTL (would think the whole chip be blown), was more thinking bad trace/via.
The Broken leg is one for the books ! specially from a chip in that location.
added: I would buy two 74f151's since they are not expensive (shipping is more ;p ) so you have two new exact matched ones.... just a thought.

Horun wrote on 2021-05-02, 23:34:

I did not suspect a blown gate on TTL (would think the whole chip be blown), was more thinking bad trace/via.

It doesn't have to be completly dead, just degraded enough to not pass the signal in time to output during the ABC input sweep. 74F chips run pretty hot - it's still bipolar tech, but that's the price in general for fast operation. They can just die due to heat and over/undershoot stress even if there weren't any actual excursions outside absolute maximum ratings. On these mobos the F chips are usually used in clock circuits, cache glue and ISA buffers/glue. I've seen those die in all these places, and it's never easy to figure out.

nice! good job everyone tracking this one down

Wow! I've been following along with this one and it's seriously impressive how you guys were able to hunt this down and figure out what was wrong. I'm glad your able to get the parts and save your board as well.

CelGen wrote on 2021-05-02, 22:58:

Edited: Oh, nope I guess the leg fell off again. Resoldered it and both floppy and IDE are working again. Currently testing for stability but I think we are good.

Nice, thanks for sticking with it, I learnt a fair bit. Was there any sign the problem chip had been replaced or worked on before? It might be the angle in the photograph, but the marking is much clearer on that one, compared with the other chips.

I just had a look and can't see this version of the board listed on UH19. The closest I could find (searching for 85C461 chipset, 486SV2) is the VL/ISA-486SV2 Rev 1.72: http://www.win3x.org/uh19/motherboard/show/821 but that has no photo to compare with. You took a good photo right at the top of this thread, so maybe it can be added if it isn't already there.

Interestingly(?), there's a '460 chipset version with VLB slots (Rev 2.4), and compared to your Rev 3.14, the photo shows an extra logic IC between the two slots: http://www.win3x.org/uh19/motherboard/show/822, maybe because the '460 doesn't quite support VLB. Wikipedia does list the '460 as supporting VLB, which I might change to saying 'Partial' with a reference to page 3-12 in the datasheet, if a) no one objects (after all, there are '460 MBs with VLB slots) and b) I remember to get round to it.

I'll have a look at the entries and add stuff where applicable, thanks for the heads-up.

Much rejoicing !

so all it was, was a 74F151 which wasn't relaying the DRQ2 signal to the chipset anymore . Correct ?

weedeewee wrote on 2021-05-03, 10:20:

Much rejoicing !

so all it was, was a 74F151 which wasn't relaying the DRQ2 signal to the chipset anymore . Correct ?

Seems to be. It looks like non-DMA communication was possible for controlling the drive (so spin-up and head positioning worked) and for reading FDC status registers (so the IMD alignment test worked ok with a formatted disk). The bus test card must trigger on the DRQ line, not the DACK, so it showed DMA2 active. Plenty of people said DMA2 could be the problem, and you asked what the test card was actually triggering on, so there were clues. I didn't know enough about the bus test card, DMA or how the FDC must use DMA only for sending actual sector contents, to realise that those two things working (IMD and bus test) didn't mean the DMA channel was ok.

It was only the testing showing DACK2 not changing that was important, and led to trying to check what the DRQ2 input was doing going to the chipset. But the chipset multiplexes several input signals to a single pin and the speed was too fast to ID the DRQ2. So that led to testing by swapping parts around. It looks like the 85c461 chipset multiplexes the DRQ lines similarly to that seen in the 85c471 datasheet, which shows one of the three multiplexed inputs used for De-Turbo, WIRQ, RC, IOCHCK and DRQ3..0. Two other multiplexed inputs handle the other 3 DRQ and 13 IRQ lines, so the chipset just needs 3 output and 3 input pins to handle 24 inputs (frees up pins for cache control). The 85c460 datasheet doesn't cover the multiplexing configuration detail, and no one's found a datasheet for the 85c461. But the '460 one does show a diagram on page 3-3 for the connections of a cached system, showing the IRQ and and DRQ lines multiplexed. So it probably follows the same scheme shown in the '471.

I just had a look and can't see this version of the board listed on UH19. The closest I could find (searching for 85C461 chipset, 486SV2) is the VL/ISA-486SV2 Rev 1.72: http://www.win3x.org/uh19/motherboard/show/821 but that has no photo to compare with. You took a good photo right at the top of this thread, so maybe it can be added if it isn't already there.

Go for it. I got no problems with people using those.

Glad to know that it was just a TTL at fault, and not an ASIC.
As I mentioned, they are getting to that age.

Dear all,

I have a similar situation here:

- floppy drive content can be read but not executed. System simply hangs (warm) or a core hardware bios based message appears: parity error and system fully locked
- tested with two fdd‘s
- same fdd‘s work in another pc

Assumption:
- I by incident overclocked the DMA clock to 14MHz for some time. It should be at 4MHz!!
- this could have destroyed sth on my isa controller card for hdd/fdd

Remarks:
- hdd does still work
- other irq and DMA related activities do still work as eg SB16/NW etc.

That’s why I assume controller card.

Remarks are really welcome 😀

Rgs

What do you have connected as HDD in that system? A CF card? Did you try the floppy boot with no HDD of any kind connected? Just in case disconnect any cables and adaptors/sockets as well, leave only floppy drive and it's cable connected.

It’s a normal 420mb plus 2gb hdd.

Ok thanks I will try this. I already tried at least without soundcards with the same outcome.

Thanks a lot.
Issue identified.
After removing all cards but ide/fdd controller and vga card the system wouldn’t boot at all. Only when I removed the large hdd cable wise. Yes indeed.

Anyway I switched hdd order then it booted again 😀)

Floppy worked. As soon as i install me sb16 with an ide interface floppy boot fails. The sb16 is the root cause.

Thanks guys!