486 motherboard compatability with newer PCI video cards - Page 1
Forum section: electronics / general

author: poot36
author_rank: Frequent Contributor
timestamp: December 05, 2020, 07:04:54 pm
content: I have a late model 1995 MS-4144 micro-star 486 motherboard with PCI slots and I am trying to get a PCI ATI Radeon 7500 64MB video card to work with it. It is a universal PCI card (supports 5V and 3.3V signal levels) and when I first plugged it in I had no video and just POST beeps indicating no video card detected. I found out this was because the card needed 3.3V power to operate so I used the 3.45V power from the CPU VRM to apply power to the 3.3V pins on the PCI slot. This got me a display but now the system partially hangs on the part of the BIOS screen that says Wait... (this is where it would detect the hard drives and I think the floppy drive). I can get into the BIOS and change settings and did disable the hard drive controller and floppy drive controller but it still partially hangs on the Wait... prompt (if I do wait it does eventually beep twice and that is it. Got any ideas why this is? The chipset on the motherboard appears to support PCI version 2.0 and the video card could support PCI version 2.2 based on the date it was released (late 2000, early 2001). I have attached the datasheet to this post if that helps, it is for the SIS 85C496 and SIS 85C497 chipsets.

author: dexters_lab
author_rank: Unknown
timestamp: December 09, 2020, 08:45:59 am
content: i wouldn't have thought the VRM on the board would have enough grunt to power both the CPU and whatever parts are 3v3 on the Radeon.I would try getting yourself an ATX -> AT power adaptor cable and use an ATX PSU (one of the earlier pre ATX12V would be best suited here), but it does mean you will have a dedicated 3v3 line you can use to supply power to the PCI ports and see if that makes any difference.You should also try some other PCI cards in there and see what works and what doesn't, it might give you a clue as to the issue

author: poot36
author_rank: Frequent Contributor
timestamp: December 10, 2020, 02:57:58 am
content: I did monitor the 3.45V line and I did not see any dips but I could check it with a scope. If I installed a supported video card and booted the system it would detect the ATI card but give an error about resources being in use so that it could not enable multi monitor mode. I also tried a TV tuner card in that slot and it detected it but I did not try loading any drivers for it. Do you think a BIOS update would fix this issue as my BIOS is from 1994 and the latest one is from 1996? I have a POST diagnostic card if you want me to try that in the system with the error'ing video card if that would help. I did read in the chipset datasheet that the PCI slots share the bus with the IDE connection and I am wondering if when it goes to look for the hard drive (even if I disable the hard drive controller) it winds up sending some sort of PCI message the video card does not like and that freezes it. The CPU VRM is just a linear voltage regulator rated to 2A of current (with a heatsink that is not installed) running from the 5V PSU line with a 0.15V or so diode drop on the input to the regulator. It does not get hot and has thermal shut down so I would like to think that if it was the issue the screen would turn off.

author: poot36
author_rank: Frequent Contributor
timestamp: December 14, 2020, 12:38:58 am
content: Got it working with a bios update. Thanks for the help.