Try a different drive and cable just to be sure. Older 1.44MB still had drive select jumpers. Ensure it is set to the second selection (DS1 if numbering starts from 0, or DS2 if numbering starts from 1).
It is expected that setting only the B: drive in BIOS will not allow booting from it. It really has to be the A: drive.

l.neidlinger wrote on 2024-08-13, 01:25:

I've dug up this old Pionex 486 system and have been trying to get it running. So far it has been rebuffing all my advances. […]
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I've dug up this old Pionex 486 system and have been trying to get it running. So far it has been rebuffing all my advances.

The motherboard is a Biostar MB1433/50-AEA-V. For some reason it is refusing to see the A:\ drive.

I replaced the Dallas clock chip with a new drop in replacement that uses a coin cell. It now keeps bios settings just fine. I have drive A: set as a 1.44m 3.5 floppy, and the drive is plugged into the end connector, past the twist in the cable. However in this configuration I get absolutely nothing from the floppy, it doesn't try to seek or anything.

I tried swapping out the cable for a known good one, no change.

If I set drive A: to not installed, set drive B: to a 1.44m 3.5 floppy and plug the drive into the drive B: plug on the cable, I get a seek out of it, but still get a "Diskette Boot Failure Insert Boot Diskette into A:" and it will refuse to read from the B: drive

If I set drive A: to a 1.44m 3.5 floppy and set drive B: to a 1.44m 3.5 floppy with both actually installed, I get an "A: drive error" during post, followed by the same Diskette boot failure error as above after hitting F1 to continue.

I tried replacing the Super I/O card with a Promise DC200 I had laying around that has a floppy controller, but I get the exact same behavior with that card, in addition to the HD controller not wanting to work with it either. The HD controller in the super i/o card seems to work just fine and detects the 1gb CF card no problem. I have tried the above with the CF card removed as well.

Does Your floppy controller has some jumpers for regulating is it a: or b: drive? Try finding jumper setting for this controller and search what does jumpers are for. That could be a start....

Tried a few different cables, no success with those.

The original drive that came with the system does have jumpers, but it was already set as DS1. I also tried a newer drive and got the same results. It is a much newer drive, though, and it is missing about 1/2 the pins in the connector, so I don't know if the controller would play nice with it.

I pulled a drive out of another similar vintage machine, and it immediately worked. I guess the original floppy drive has died, and the newer drive I tried doesn't play well with the controller.

l.neidlinger wrote on 2024-08-13, 14:51:

The original drive that came with the system does have jumpers, but it was already set as DS1.

DS1 out of? Some of the drives start the numbering from 0, in which case DS1 is correct, but some start with 1. On such drives you want DS2. In other words for a PC compatible you always want 2nd number in sequence, whatever it might be.

Deunan wrote on 2024-08-13, 16:22:

l.neidlinger wrote on 2024-08-13, 14:51:

The original drive that came with the system does have jumpers, but it was already set as DS1.

DS1 out of? Some of the drives start the numbering from 0, in which case DS1 is correct, but some start with 1. On such drives you want DS2. In other words for a PC compatible you always want 2nd number in sequence, whatever it might be.

The drive numbering starts at DS0.

Does your mainboard show any sings of oxidation from leaked varta battery? Maybe some trace under the ISA slot is damaged? Have you tried the Floppy Controller in other ISA slot?

On the other side, to rule out the conflict between other cards in your system, I would suggest to remove everything from your system except Graphics card and your floppy controller. Additionally it's worth to load default settings in BIOS. With such config try to get booted from Floppy Drive.