Your can measure the "CLK" pin with your Multimeter if its support frequency measurement in this range - my very cheap one does.

konc wrote on 2020-02-07, 14:43:

If I set it to the fixed value 14.318/2 (which is less that 8MHz!) the PC doesn't boot.

How would a software utility to read the ISA bus speed help in that case? But if that option doesn't work, the 14.318 MHz quarz crystal on the mainboard is probably faulty. You may not notice, as it's not used for much in anything post-XT.

Anyway, with an oscilloscope, you can probe pin B20 (or was it B30?) on the ISA bus to see how fast it is driven. I don't think this can be done using software.

B20 is the correct place, B30 is just the fixed 14.318 oscillator output

I see, thanks guys for the responses. I also didn't expect for some software to be able to measure this. My multimeter doesn't measure frequency but I'll borrow a friend's to check, mostly because of curiosity. Also thanks for pin location, you saved me some time 😉

derSammler wrote on 2020-02-07, 15:00:

How would a software utility to read the ISA bus speed help in that case?

It can't but because of this I'd check other options as well, like is CKL2/10 really 8MHz?

derSammler wrote on 2020-02-07, 15:00:

But if that option doesn't work, the 14.318 MHz quarz crystal on the mainboard is probably faulty. You may not notice, as it's not used for much in anything post-XT.

If I understand you correctly the 14.318 oscillator is used only when you set the fixed value option in BIOS and in other cases (CKL2/x) the 80MHz one is used? That would explain the behavior. Is it used for anything else? I think I have a spare one from a dead motherboard, I might try to change it since it's location makes it really easy.

Edit: oh shit wait! Is the 14.318 osc used for the floppy controller? That would explain some weird issues I'm having with all floppy drives/IO cards/cables I've tried (randomly the floppy doesn't read/write/try to do anything at all)

It could be that the clock for your DMAC is derived from that xtal, which would explain the floppy issues

Is it used for anything else?

It was mainly there originally for CGA and the color burst frequency was derived from it. A few later ISA VGA cards used it as well and only show a black screen when it's missing. It wasn't mandatory on AT-class mainboards.

If your BIOS has that option to use it for the ISA bus clock, it's quite possible that it is used for other things as well.

Hey guess what! It was a faulty oscillator 😃 Although my attention was towards the bus, I would have never reached the oscillator myself without derSammler's observation

derSammler wrote on 2020-02-07, 15:00:

But if that option doesn't work, the 14.318 MHz quarz crystal on the mainboard is probably faulty. You may not notice, as it's not used for much in anything post-XT.

After confirming that it might also be used for the FDC I couldn't wait anymore and butchered an old vga to replace the crystal. Now the PC boots with the fixed 14.318/2 value and the floppy worked without quirks for more than it has ever done. Now I need to order a couple of new crystals to restore the donor card.

Again, thanks everyone for this. Not only this thread has been addressed (no software to check ISA speed but it can be probed with a multimeter/oscilloscope) but also an issue that would have driven me nuts was solved.

Nice. 😀