CelGen wrote on 2021-04-25, 22:53:

I put together an ASUS ISA-486SV2 Rev. 3.1 motherboard with a Trident VLB video card, PRIME 2 chipset disk and I/O ISA controller with a 66mhz DX2 and I can't make it work with the floppy drives. Put a bootable floppy in and while it will light the drive and spin the motor it will just sit there with DISK BOOT FAILURE.

The PSU, CPU, ram, cards, cabling and the floppy drives were tested on a different 486 board and all verified to be working. Even used all the hardware to install DOS onto a DOM with no read or write issues.

Can you post a good picture of your board and tell us what board you tested the parts ? It sounds like a resource issue or bad DMA controller (the floppy requires IRQ6 and DMA2, you can run video and HD w/o DMA on old boards but floppies require it).
Being a "single" chip chipset you need to look over that SIS 85c461 carefully to make sure no pins shorted/bridged/broken....
edit: fixed my error

Do the COM ports work? (test with a serial mouse)
If the COM ports don't work either, then, barring mis-set jumpers someplace, odds are the crystal driving the Prime 2C's baud generator probably failed. This crystal should drive both the COM ports and the floppy controller. On my Prime 2C card, the crystal is 24 MHz.

Eep386 wrote on 2021-04-26, 01:50:

Do the COM ports work? (test with a serial mouse)
If the COM ports don't work either, then, barring mis-set jumpers someplace, odds are the crystal driving the Prime 2C's baud generator probably failed. This crystal should drive both the COM ports and the floppy controller. On my Prime 2C card, the crystal is 24 MHz.

Excellent point ! If the 14.318Mhz crystal is bad it can cause similar issues also IIRC.
Added As last resort: Cannot find the 461 datasheet but the 460 datasheet is out there. Would also check with a DVM the ohms from ISA slot to Chipset on these: the 460 and 461 may have same pinout
IRQ6: ISA B22 to 460 pin 129
DMA2 REQ: ISA B6 to 460 DMA Request 2 pin 149 DRQ2
DMA2 Ack: ISA B26 to 460 DACK2 to pin 191 DACK2
if those show hi-res (greater than 1 or 2 ohm) then a board issue, check the via's. The 461 may have diff pinout but assumes is same as the 460.
http://www.bitsavers.org/components/sis/85C460.pdf
Someone more knowlegable please correct me if I errored, thanks !

Good news I guess, I opened Procomm and setup my serial tester in loopback mode and even at the highest baudrate both ports worked fine.

Here's a photo of the board. *Go here and right-click to view the full-size image* Aside from a few more green crusties from the battery cleanup it's in excellent condition with no chip damage or trace rot.

Here's a photo of the board I tested in. As per this thread it's a no-name with a slightly different layout than found online. I didn't even know if it worked. It was just close by when I needed something to test with. *Click here because again it's a big image*

Is it possible that somebody swapped out the BIOS chip and the board doesn't actually have the correct BIOS?

Try a different I/O card. I had to junk a few I/O cards because the IDE and floppy ports wouldn't work no matter how I set the jumpers. Guess they do go bad...

Horun wrote on 2021-04-26, 01:33:

Can you post a good picture of your board and tell us what board you tested the parts ? It sounds like a resource issue or bad D […]
Show full quote

CelGen wrote on 2021-04-25, 22:53:

I put together an ASUS ISA-486SV2 Rev. 3.1 motherboard with a Trident VLB video card, PRIME 2 chipset disk and I/O ISA controller with a 66mhz DX2 and I can't make it work with the floppy drives. Put a bootable floppy in and while it will light the drive and spin the motor it will just sit there with DISK BOOT FAILURE.

The PSU, CPU, ram, cards, cabling and the floppy drives were tested on a different 486 board and all verified to be working. Even used all the hardware to install DOS onto a DOM with no read or write issues.

Can you post a good picture of your board and tell us what board you tested the parts ? It sounds like a resource issue or bad DMA controller (the floppy requires IRQ6 and DMA2, you can run video and HD w/o DMA on old boards but floppies require it).
Being a "single" chip chipset you need to look over that SIS 85c461 carefully to make sure no pins shorted/bridged/broken....
edit: fixed my error

Yep. DMA problem would also be my guess in this case, probably damaged traces.

It might sound silly since you have tried the floppy cables on another board, but do try different cables on this one

Try a different I/O card.

I've tried five different cards.

It might sound silly since you have tried the floppy cables

I've change the cables twice.

The drive, cables and controller are tested working.

Is there an ECP DMA jumper setting for the board?
I've met a few VLB boards with jumpers for the ECP DMA, even though they didn't feature built-in I/O beyond the keyboard controller.

Not that I can identify.

So I beeped out DRQ2 and DACK2 and they both seem to be intact spanning across the bus. DRQ2 originates from U8 pin 13 (74F151BN) and DACK2 comes from U24 pin 13 (74LS138N) and both chips seem to have their inputs all beep through to the 85C461, except for Z̅ on the 151. I'm not sure where it originates from so I won't chase it without a reason but it doesn't beep to the chip.

Are you 100% sure that the correct motherboard BIOS is being used? A lot of the older systems like that would work with BIOSes from other motherboards with the same chipset but that didn't mean that everything would work properly.

Could even be that the BIOS is for that model of motherboard but for a different revision.

I am not positive by any stretch that this is the correct BIOS for this revision board. I do not have an EPROM image to burn and compare against.

So I remembered that I have a POST card that latches indicators when IRQ's and DMA's are triggered and after verifying that I should see IRQ6 and DMA 2 when the system goes to boot from the floppy drive using our test motherboard I tested the Asus and sure enough I'm seeing IRQ6 but no DMA.

EDITED: Scratch that. The POST card was not fully seated. We do have DMA 2.

CelGen wrote on 2021-04-27, 01:45:

EDITED: Scratch that. The POST card was not fully seated. We do have DMA 2.

Did you try disconnecting the HDD/SSD/CF card and setting NONE in setup? I had a few problems with BIOS reporting HDD/floppy failure with specific CF cards on older mobos. With the card removed mobo booted properly from floppy. I eventually resolved that by using older CF card of different brand.

I'm not using a CF card. I'm using a DOM.
Anyways for the purpose of testing the entire IDE ribbon cable is unplugged and only one floppy drive is attached and configured in the BIOS.

What happens if you try to format a floppy, with no command line switches? Does it say what capacity it's going to try before failing? Had an odd case on another thread where the BIOS menus had the correct settings, but the registers ended up with the drive as a 360k 5.25".

What's that floppy track test program people generally use ?
Is it ImageDisk by Dave Dunfield ?

weedeewee wrote on 2021-04-27, 16:50:

What's that floppy track test program people generally use ?
Is it ImageDisk by Dave Dunfield ?

That's what I used (IMD) when I was having problems with a drive. Handy rough checking of RPM and alignment.

Odd one this. From the first post, all the hardware has checked out ok with another motherboard and multiple interface cards and cables have been tried on this motherboard with nothing working. I think that means the FDC on the interface card works with the drive, and the drive is aligned ok. Power connector to the drive ok? So the common flaw seems to be the motherboard, but everything else works ok, so it's not like there's a major bus issue. I'm assuming the interface cards have been tried in different slots? The interface card turns on the drive motor, so the fdc is getting some instructions, but then it sounds like nothing comes back. That gets deeper in to bus stuff that I don't know about, so I thought it worth checking whether the computer was trying to access the drive as something other than what the BIOS setting was (i.e. not a bus issue).

Does the drive head ever step (e.g. when doing a step to track 0 on startup)?

Hmm... starting to think a chip or other component on the mobo went bad, either a TTL or (hopefully not) one of the main ASICs.
They are getting / have gotten to that age.