SIS 496 PCI Motherboard - working but only with graphics card in third PCI slot

[dicky96]

So I have this 486 DX4 100 on a SIS 496 motherboard. And it works - but only if I put the graphics card in the PCI slot furthest from the CPU. I didn't try a sound card or IO card in the other slots, mainly because I don't have a hard drive of small enough capacity this would work with, unless I could persuade it to work with a larger more modern IDE drive. Therefore I don't have an OS installed.

All the PCI slots look OK and I don't see any other obvious damage. Could it just be something in the BIOS? It would be nice to save this from the trash as there are not so many around any more. And it is somewhat collectable these days.

I hope some of you have more experience of such old hardware.

[dicky96]

Re: SIS 496 PCI Motherboard - working but only with graphics card in third PCI slot

Ahhhh... OK I got it working. I was going to meter out all the PCI slot pins according to this guide

https://www.slideshare.net/neilfarle...y-pconlinever2 (slide 17/45)

which explains which pins are in parallel on all slots, and how the other ones are connected - so it should be useful for troubleshooting a faulty slot.

However just before I started I then had the bright idea of putting my POST diagnostic card in each PCI slot to see what it did, and I found I got the same sequence regardless which slot I used. A bit puzzled why they behaved differently (slot 1 and two gave the typical beeps for missing graphics card) I put the VGA card back in and found it now works OK in slot 2 and 3, and intermittently in slot 1. I tried reinserting it a number of times in slot 1 and that didn't help too much but I did the same reinserting the POST card a number of times and slot 1 now also seems to work OK

I also found a (slightly noisy) CPU fan for the heat sink, the original one was seized up.

So to continue I think I will now try to see if I can get it to recognize an IDE HDD so I can try installing Windows 98 from CDROM (which I have - but I only have 8Mb or RAM so maybe that isn't going to work). Or Windows 95 but I read that is more tricky without a bootable DOS disk. But I do have floppy drives and one floppy disk.

Rich

[momaka]

Re: SIS 496 PCI Motherboard - working but only with graphics card in third PCI slot

Nice save, dicky96!

Yeah, on motherboards this old, you can expect the connectors to be a bit tarnished at this point. In fact, some cheaper motherboards didn't have really good moisture protection on both the connectors and the traces, so do check both sides of the board carefully for any possibly corroded traces. Some old boards like these also had supercaps or rechargeable batteries to keep CMOS, and they would leak and destroy contacts too... though yours looks to be safe from that.

Also, remove and reinsert any jumpers too - they may have developed bad contacts too from sitting for so long.

Last but not least, maybe give Windows 3.11 a try?
I'm not an expert in that area though - anything older than Windows 98, I do remember using but was only a small kid, and so I've never really learned that oldschool stuff too much. I think the oldest PC my family had was running Windows 3.11 and DOS 6.x on a black&white screen. I remember playing Commander Keen 4 on it and a bunch of other DOS games... but that's about it.

[dicky96]

Re: SIS 496 PCI Motherboard - working but only with graphics card in third PCI slot

Hmm yeah I'm not quite out of the woods yet with this one. I had problems getting it to autodetect a hard drive. The smallest one I have is 20Gb and it would detect that (as 8.5Gb - which is OK as this 486 board seems to be a late model that supports LBA) but after trying to get it to detect a CDROM, I can't get it to detect any IDE device anymore.

I have a thread about this on Vogons forum if you are interested as it seemed a more relevant place to ask seeing as I thought it was BIOS related but now it seems I may have broken something

Obviously this forum is *the* place to be for electronics repair but Vogons is also a nice forum for vintage computer hardware and plenty of guys with that very specialized knowledge there (so I hope this link isn't considered spam)

https://www.vogons.org/viewtopic.php...934669#p934669

[momaka]

Re: SIS 496 PCI Motherboard - working but only with graphics card in third PCI slot

Obviously this forum is *the* place to be for electronics repair but Vogons is also a nice forum for vintage computer hardware and plenty of guys with that very specialized knowledge there (so I hope this link isn't considered spam)

https://www.vogons.org/viewtopic.php...934669#p934669

No sir, not at all!
It's all about sharing knowledge and information. Badcaps, EEVlog, AudioKarma (AK), Vogons, and a number of other forums are still the best source of tech/geeky knowledge on the net, IMO.

I've ran into Vogons many times when looking up old hardware stuff, but never really knew what its "theme" was, so I never really stuck too much there. But now that you mentioned it, it seems like a great resource for old/retro PC stuff, so will definitely keep it in the back of my head to look up things there if I ever get stuck on some old PC project.

That aisde, I just finished reading your thread on there (after getting stuck for a millenium in the GPU and old hardware forums, lol)... and yeah, see if the CF card works. If not, you may have an IDE controller issue on your board, or perhaps noise on the power rails. On that note, what PSU are you using to power that system and did you check it for bad caps (even if they look good)?

[dicky96]

Re: SIS 496 PCI Motherboard - working but only with graphics card in third PCI slot

Oh yeah - The PSU. It came with the motherboard, CPU, RAM, SVGA card (can't really call it a GPU) and MultiIO with cables. There was no AT case or anything else. It just came in a cardboard box of bits at the flea market and I am pretty sure I paid no more than €10 for the lot. The sound card is one I had lying around for a while, from the same flea market - condition unknown as of yet.

Anyway I fixed the PSU last year (though I don't remember what was wrong with it now) and all the caps look good - not bulging - but not checked other than that. I will check for noise on the voltage rails on my scope tomorrow. Anyway there are not so many caps so I could change them. The mobo originally worked (well it booted to the BIOS screen) once I fixed the PSU but with the problem with the PCI slots and I left it on one side for maybe a year.

Other than that yeah, I may have damaged the IDE controller. Something definitely changed after attaching the CDROMs to find a working one, but having said that all the CDROMs, the HDD and the same IDE cables have been on another rig to test since, and no damage occurred to that one, though two of the CDROM are now in the trash.

[dicky96]

Re: SIS 496 PCI Motherboard - working but only with graphics card in third PCI slot

OK. So I got a 2Gb CF card and an IDE-CF adapter... and the motherboard will will not detect an IDE device is attached

Bear in mind the board originally detected my 20Gb HDD as an 8Gb HDD, but for some reason after I tried to detect a CDROM it will not detect anything any more. The HDD and CDROM work OK on another PC, or at least non of the CDROMS I tried damaged the other PC though I threw two of them away for not reading disks properly.

I will try the IDE-CF adapter on another motherboard on Monday to check it is actually working but it was new so I guess it will be

Anyway I had a look around the 74F244 tri-state buffer drivers that connect between the IDE connectors and the 85C496 Chipset but I couldn't really work out what I was supposed to be seeing on my scope when trying to detect a Hard Drive. it's like only one side of the buffers has any data, the other side is floating, even though one of the EN pins is enabled on each buffer IC. Also only 14 of the buffers have data signals on one side - the other two don't.

Again I'll post some more details on Monday, but for now I thought I would google to find out how IDE actually works. Based on the premise that if I know what is supposed to be happening it will be easier to work out what is not happening

I can find a pinout for the 40 pin connector, but when I google for 'how does IDE detect a Hard Drive' is not telling me what I want to know

So I would have thought this was easy enough to find out, but can someone tell me - How does IDE detect a hard drive's parameters? What happens to make it do that?


Rich

[dicky96]

Re: SIS 496 PCI Motherboard - working but only with graphics card in third PCI slot

The good news is, it's working

But I am not exactly sure why

I tried my 2Gb IDE-CF on a test rig and it detected just fine. So I put it back on the 486 on secondary IDE this time, and it didn't detect. Then I did BIOS defaults, saved that, and it started to work on both IDE secondary and Primary.

This is I did the BIOS defaults several times while trying to get the 20Gb drive to recognize, after it stopped detecting, but the 486 board was having none of it. Even after I used Seatools to set the 20Gb to 2Gb

Still, the 486 DX4-100 lives again

[momaka]

Re: SIS 496 PCI Motherboard - working but only with graphics card in third PCI slot

Still, the 486 DX4-100 lives again

Glad to hear you got it working!

I guess when in doubt, just reset CMOS again and again.
On the other hand, I wonder if it's a flaky BIOS or CMOS chip. I do have a few older motherboards (though not this old) that fit this failure profile - i.e. things didn't work until CMOS is reset once in a while.

[dicky96]

Re: SIS 496 PCI Motherboard - working but only with graphics card in third PCI slot

Glad to hear you got it working!

I guess when in doubt, just reset CMOS again and again.

Yeah I guess that is true - though didn't some famous genius say that the definition of insanity is doing the same thing over and over again expecting a different result?

[cpt.charlie]

Re: SIS 496 PCI Motherboard - working but only with graphics card in third PCI slot

Maybe you have a bad PATA cable, those used to be a bit problematic specially if you are using the original ones!

[ChaosLegionnaire]

Re: SIS 496 PCI Motherboard - working but only with graphics card in third PCI slot

However just before I started I then had the bright idea of putting my POST diagnostic card in each PCI slot to see what it did, and I found I got the same sequence regardless which slot I used. A bit puzzled why they behaved differently (slot 1 and two gave the typical beeps for missing graphics card) I put the VGA card back in and found it now works OK in slot 2 and 3, and intermittently in slot 1. I tried reinserting it a number of times in slot 1 and that didn't help too much but I did the same reinserting the POST card a number of times and slot 1 now also seems to work OK

Yeah, on motherboards this old, you can expect the connectors to be a bit tarnished at this point. In fact, some cheaper motherboards didn't have really good moisture protection on both the connectors and the traces, so do check both sides of the board carefully for any possibly corroded traces.

Also, remove and reinsert any jumpers too - they may have developed bad contacts too from sitting for so long.

next time, try electrical contact cleaner spray for cleaning dirty electrical contacts. sometimes, spraying and leaving the contact cleaner to dry multiple times is needed to clean out dirty contacts so that they work properly again may be necessary.

Last but not least, maybe give Windows 3.11 a try?
I'm not an expert in that area though - anything older than Windows 98, I do remember using but was only a small kid, and so I've never really learned that oldschool stuff too much. I think the oldest PC my family had was running Windows 3.11 and DOS 6.x on a black&white screen.

yea, dicky, i recommend ms-dos 6.22 as the base os with windows 3.11 installed on top of that. thats the best combo. u get the best of both worlds, a commandline os and a gui based os so u technically end up with a dual booting system: commandline or gui to use as u please.

I remember playing Commander Keen 4 on it and a bunch of other DOS games... but that's about it.

oh boy, some good ol' memories there. i played so many dos games back in the day, i cant remember them all. i played both keen 1 and 4, lots of id software and apogee software games like wolfenstein 3d, cosmo's cosmic adventure and many others like moraff's revenge etc. also had educational software games from the learning company too.

I had problems getting it to autodetect a hard drive. The smallest one I have is 20Gb and it would detect that (as 8.5Gb - which is OK as this 486 board seems to be a late model that supports LBA)

it looks like those hard disk size detection issues are due to the bios being unable to detect the hard drive parameters properly. the 8.5gb limit u are getting is typically of the addressing limits of the chs addressing method. try going into the bios and set the hard drive parameters manually and saving the settings. i remember bios software back then wasnt too smart and u had to set the hard drive parameters manually in the bios or else the hard disk wont be detected properly or it becomes not useable by the os.

also make sure the cmos battery holder contacts are not tarnished and the battery holder legs are making good contact with the through holes on the mainboard.

lastly, thats a really sweet dos retro rig u got there! one of the fastest 486 cpus at 100 mhz, lba hard disk support, a pci video card, u need a soundblaster classic for the isa slot and 2 more sticks of ram to max out the ram at 16mb and u're good to go!

[momaka]

Re: SIS 496 PCI Motherboard - working but only with graphics card in third PCI slot

yea, dicky, i recommend ms-dos 6.22 as the base os with windows 3.11 installed on top of that. thats the best combo. u get the best of both worlds, a commandline os and a gui based os so u technically end up with a dual booting system: commandline or gui to use as u please.

Yup, I think that's what ours was back then exactly. Booting the PC, I remember getting to a CMD on the screen. After that, my mom had thought me the commands to type to start my games (including Commander Keen, among others) or Windows 3.1. The funny thing is, I wasn't even old enough back then to be in school, nor did I know how to read/write fully (I could read/write words maybe, but not full sentences) as I was like 5-6 YO (not to mention English isn't even my native language.) Kind of bizzare, though, that to this day I still remember some of those commands.
I guess I was just meant to be a computer nerd, lol.

oh boy, some good ol' memories there. i played so many dos games back in the day, i cant remember them all.

Same.
It's even harder for me, since like I mentioned above, some of these games I played before I learned to fully read/write (and not even in English, for that matter.) So trying to recall what some of these games were called more than 25 years later... yeah, gonna need the help of the internet for that.

also make sure the cmos battery holder contacts are not tarnished and the battery holder legs are making good contact with the through holes on the mainboard.

Yup, I've had a board with loose contacts like that. Threw me in a loop to try to figure out why the settings kept going out when the PSU was unplugged. Eventually found one of the CMOS battery's connector was just barely not touching the battery terminals... hence the loss of CMOS every time.

and 2 more sticks of ram to max out the ram at 16mb and u're good to go!

Would be funny to tell that to a young person today. They'd be like "oh, you had 16 GB back then? Nice!" *Ahem* No, 16 Megabytes.