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weird 486 motherboard problem

J

jh1523

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coastal New England
It's the first time I run into something like this, and I'm a little bit stumped.

I acquired recently a 486 motherboard, here it is on TH99:
http://stason.org/TULARC/pc/motherboards/A/ACER-INC-486-VI15G.html#.U4uGHIXeLFk

I am having the following problem with it: I can't make any FDD controllers work with it. I have a sizeable stack of FDD/HDD/multi-io cards, both ISA and VLB. I have tried all of them (more than 15 cards) in this motherboard. I even tried an Adaptec AHA-1542CF. No matter what card I try, I get one of 2 results:
1. error message "FDD controller failure, press F1 to resume" - after F1 is pressed I get the POST summary screen, then it hangs
2. no error message, shows POST summary screen then hangs

This is with a known good 3-1/4 floppy drive with a known good boot disk in it, correctly powered/jumpered/connected through a known good floppy cable etc. All the controller cards have been verified working correctly in another system, the floppy drive and disk similarly verified etc.

The board has an AMI graphical BIOS. At first I thought it may have been a conflict between the adapter card BIOS and shadow RAM. I have since disabled all RAM shadowing I could find in the BIOS, no luck though. Same problem persists. Even the AHA1542CF card gives me the same problem, "FDD controller failure" then hangs. And this card also works correctly in another system.

I even tried the motherboard's built-in "safe CMOS settings" and "failsafe CMOS settings", and none of this helped.

I am out of ideas at this point. The mobo POSTs but can't boot from floppy.
 
Could you describe any other cards installed in the system, CPU and clock speed, and memory? More information might remind someone of a special conflict between components.

Some of the possible causes I can think of:
Jumper mistmatch.
Bus speed is too high. Test by lowering CPU speed to 25MHz which should get the bus speed lower as well.
Cache chips have failed. See if you can boot after turning the cache off in BIOS, if the BIOS lets you.
 
No onboard I/O controller. That is really weird.

I agree to try booting without the cache.

Its worth noting that decreasing the CPU FSB may not decrease the ISA bus. On my Acer AP43 with SIS496 and graphical BIOS, the ISA is locked to 7.2 MHz by default
 
I used a generic Trident TVGA8900 ISA VGA card and nothing else. The CPU is a 486 DX2/66 (also tried a DX-33, no change). Memory is some generic 72-pin FPM SDRAM, 2 sticks of 8M each. I think they're 4Mx16 or somesuch.

Will try disabling the cache and see if that makes a difference. BTW: why would disabling the cache work?

BRB.
 
Last edited:
Disabling the external cache doesn't change a thing.

The memory is IBM 70ns FRU: 92g7321 72-pin. It's EDO, not FPM, 2Mx32.
 
Under what circumstances did you obtain the board? It could be hosed.

Try putting a hard drive in and boot from that. Then you can run some diagnostics. Make sure the diagnostic program(s) is (are) on the drive before you hook it up to the iffy board. :)
 
Have you tried clearing the CMOS and setting up again, I have an VI12G mobo with an Award bios and had a few problems with that when i first got it but after removing the battery and leaving it out overnight i stuck a new battery in re-setup and it's been working fine since, also make sure the floppy drive is setup properly in the bios or set to not installed if the controller card has it's own bios.
 
The CMOS was cleared at least once recently. I got the board off ebay, it looked in good state in the pictures. It came without a battery, no apparent leakage damage either, and a few days later I added a new battery. So it was for a while without a battery.

I'll try a HDD next, but the problem with that is - the only one I have available is a 2.5" laptop one, and I don't have an adapter. See if I can scavenge another HDD somewhere.
 
Update: the motherboard boots very nicely from a DOS HDD, but only if both floppy drives are disabled in BIOS, and the hard drive is set as the first boot device. Otherwise it still hangs. It continues to boot and function correctly after I re-enabled the cache. I tested several of the controller cards, and it behaves the same for all.

I still can't explain what's happening. A 486 without any FDDs will be weird to operate.
 
The last time I had floppy issues with a 486 board, was when I was trying to enable the CPU writeback-cache (P24D jumper settings) on an AMD5x86 @ 133. It worked but the floppy was inaccessible. Switching to Intel DX4 settings worked fine. Make sure the jumpers are set for 486DX/DX2 and not for P24D. Maybe even try switching between the AMD and Intel jumper settings for DX/DX2 CPUs.
 
You could probably get it to work with an IDE ZIP drive. Think of it as a 100MB floppy. :) You'd only need one or two disks.
 
The jumpers are definitely set for DX2, not P24D as per the TH99 page I linked to above. I can try the jumper settings for AM486 too.

One other unusual thing, the POST screen reports the CPU as 486 DX2/66-S. That would make it a P24S, even though no markings are on it to that effect. Could this be part of the problem?

Stone said:
You could probably get it to work with an IDE ZIP drive. Think of it as a 100MB floppy. :) You'd only need one or two disks.
Click to expand...

Zip disks/drives are the spawn of the devil. Vade Retro, Satan! :D

(me and Zip drives go back a long way; I've had all kinds of them, IDE, SCSI and parallel. The first time I booted Linux was from a Zip disk with Slackware installed - Zipslack 3.5. But Zips have always betrayed me, so I swore I'd never touch one again :))
 
What else do you have plugged in? (port 3Fx, IRQ 6, DMA 2 must be free for an FDC to work)

If you can get ImageDisk onto that machine, there is a diagnostic mode that can deliver more information.
 
There is nothing plugged in apart from the power supply, keyboard, CPU, memory, VGA card and controller card.

More weirdness. I tried to jumper the mobo as for an AMD chip, it wouldn't POST with the DX2 chip in. Jumpered back for Inter486DX/DX2, swapped the CPU for a 486DX-33, POST but no boot from FDD, boots from HDD if FDD disabled. I swapped in the only other 486-class chip I have, an AMD 5x86-P75 (X5-133 ADW). When jumpered for AMD, it would not POST. When jumpered for Intel it does POST, recognized as a DX4-100. Boots from HDD but not from FDD, like all the other chips.
 
Seeing how you have tried 15 different I/O cards, I would consider 3 next steps.
1. Turn off everything possible in BIOS except floppy support.. Tinker with settings that Cannot be turned off after each failed boot attempt.
2. Boot to hard drive, see if you can backup the BIOS to a file on the hard drive. If the backup fails it could be a sign off corrupt firmware. (You may have to Google on how to do this per your BIOS manufacturer).
3. After a successful backup, update the BIOS if you can find a more recent version.
 
The BIOS on this motherboard is not flashable, it's in a PROM. And I've already tried turning off everything, didn't work. Turned off all shadowing, all caching, tinkered with caching policies, turned CPU/bus speed as low as it would let me (25MHz), slowed down memory timings, etc.
 
I managed to get to the bottom of the problem, and make the motherboard recognize the floppy and boot from it.

There is a setting in CMOS called "video cacheable option" - this one needs to be off. Now I'm not sure what exactly is being cached, because there is another, separate option for "video BIOS shadow" which doesn't affect the floppy operation at all. Maybe the video RAM? But that wouldn't make sense.

Anyway, just having that option off is not enough. I needed to have both a FDD and a HDD attached for the FDD to work. If I remove the HDD, the system hangs as before. That's why previously I couldn't make the system work by just playing with the CMOS options (I didn't have both types of drives attached). This is independent of the controller card, same thing happens with all the controllers I tried.

Anyway, it works now and this is how.
 
"Video cacheable" includes your video RAM as part of the system caching. If you've got issues with your cache memory, this can make a real mess. "Video BIOS shadow" copies the contents of the Video BIOS extension ROM to RAM. Both done for speed and probably not useful if you run Windows, which does not use the video BIOS extension nor does it read much of the contents of video RAM--just writes to it mostly. Pretty much there for a small speedup on DOS graphics programs.

I have no clue why either would make floppy access unreliable. Are you certain that your BIOS matches your motherboard?
 
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