Since you have all interfaces on board already why do you want to use an extra I/O controller? If you want to test the controller you need to disable all these devices in the boards BIOS.

The I/O controller is for the the few ISA/VLB boards I'm testing, which have nothing on-board...

Try disabling the onboard controller and when testing the additional controller, try a floppy drive first!

Make sure the jumpers are set correctly on that controller. You are familiar with the correct orientation of the IDE cable?

LBA does sound good for additional capacity.

These controller cards were never meant for motherboards that already included the necessary controller options.
I agree with maulwurf, disable all relevant controllers on the motherboard first.

Again, there is no such thing (IDE/FDD/COM/LPT) on the mainboard I'm currently testing the I/O card on.

The jumpers on the card enable first COM port as "COM1" and the IDE controller as "primary", and yes, I know about the proper orientation of the IDE connector (pin 1 (marked as such in the card/s PCB) = the red wire in the cable).

I'll try finding a spare floppy drive today...

Many boards have BIOS options to disable the onboard Floppy and IDE controllers, as well as COM and LPT ports.

The board has no IDE/FDD/COM/LPT connectors - does this mean it can still have on-board controllers for these?

Oh I thought it did:

ALi 1487/1489 mainboard (4PCI, 4ISA, FDD/IDE/COM/LPT)

If there are no connectors, then don't worry about it.

I should start using shorter sentences, or stick strictly to the subject...

"While I have a perfectly good [...] ALi 1487/1489 mainboard (4PCI, 4ISA, FDD/IDE/COM/LPT) [...] I'm also testing a few ISA/VLB boards."

Forget about the 1487/1489 board, I'm talking about other, older, ISA/VLB boards here. They have none of those connectors built-in, so I'm trying to use the ISA I/O card to at least have FDD, HDD and a COM port for the mouse...

Okido!

These controller cards also often have jumpers. They are quite easy, all of them to one side is on, all of them the other side is off.

Testing the Floppy is simply a recommendation because it either works or it doesn't whereas with HDDs a lot of other things can be responsible...

Roger that 😀. What about the COM port for the mouse? I currently have two ways of testing it:

1. use a DB9-to-PS2 adapter and watch if my optical PS2 mouse "lights", which doesn't happen
2. try to use an old serial mouse to navigate the "graphic mode" AMI BIOS, which again does not work

I tried to jumper the first COM port on the I/O card either as COM1 or as COM3, to no result - will try the second one (COM2 / COM4) tonight... I guess worst coincidence would be that both COM1/3 on the card and my serial mouse are broken 😀.

Not much experience with serial port stuff I'm afraid...

But I know my Logitech mouse driver auto detects pretty much any mouse, so I would just go with that...

Well, that was weird... tried it again this evening:
- disabled everything on the I/O card except FDD: FDD works
- enabled first COM port: mouse starts working too
- enabled IDE (with LBA on in BIOS): the Fireball 1.28 is detected at full capacity

The "system" even boots either of MS-DOS 6.22 and Win98SE installed on the hard-drive while in a K6-2/500 system, though incredibly slow and weird (cachechk seems to get stuck at start-up, for example) - but that will probably be the subject of a future thread, if I can't get to the bottom of it 😀.