486 AST motherboard UP8810A riser card PCI pinout found - Page 1
Forum section: electronics / general

author: poot36
author_rank: Frequent Contributor
timestamp: November 06, 2021, 04:11:13 am
content: I have managed to figure out that the bottom row of pins in the EISA riser card slot are for the 2 PCI bus card + 3 ISA breakout board. I have reverse engineered the pin-out in hopes that someone can make a PCB for this motherboard giving it PCI slots. Sorry about the poor formatting.Gnd A 1 B GndGnd A 2 B GndINTC# A 3 B INTA#INTD# A 4 B INTB#+5V A 5 B +5V+5V A 6 B +5VCLK A 7 B RST# -Gnd A 8 B Grant0Grant1 A 9 B Request0Gnd A 10 B GndRequest1 A 11 B CLK- AD 31 A 12 B Gnd- AD 29 A 13 B AD 30 -N/C? A 14 B N/C?N/C? A 15 B N/C?- AD 27 A 16 B AD 28 -- AD 25 A 17 B AD 26 -* C/BE3# A 18 B AD 24 -- AD 23 A 19 B AD 22 -- AD 21 A 20 B AD 20 -- AD 19 A 21 B AD 18 -N/C? A 22 B N/C?- AD 20 A 23 B AD 19 (Used for IDSEL (Init Device Select for each PCI slot)) -- AD 17 A 24 B AD 16 -- IRDY# A 25 B Frame# *- DEVSEL# A 26 B C/BE2# *Lock A 27 B Target Ready *** PERR# A 28 B STOP# -SERR# A 29 B N/C (Snoop done on a Pentium class riser)- AD 15 A 30 B N/C (Snoop backoff on a Pentium class riser)- AD 14 A 31 B C/BE1# *- AD 12 A 32 B PAR -Gnd A 33 B GndGnd A 34 B Gnd- AD 10 A 35 B AD 13 -- AD 8 A 36 B AD 11 -- AD 7 A 37 B AD 9 -- AD 5 A 38 B C/BE0# *- AD 3 A 39 B AD 6 -- AD 1 A 40 B AD 4 -- AD 0 A 41 B AD 2 -+5V A 42 B +5V+5V A 43 B +5VGnd A 44 B GndGnd A 45 B GndAll the pins with a - in front or behind are directly connected to other PCI devices, no buffer or other chips. IDSEL is connected to AD22 for the graphics chip and AD21 for the PCI IDE controller chip. The pins with a * in front or behind have a 33 ohm resistor connected between that pin and the bus and the pin with a ** has a 82 ohm resistor connected to the PCI IDE chip and I am not sure where else it goes (it has a 4.7K ohm pullup to the +5V rail as well). Any of the pins with a ? by them are not connected on this motherboard but on a Pentium class system they are for the 3.3V rail for the PCI slot.

author: poot36
author_rank: Frequent Contributor
timestamp: March 10, 2022, 04:06:57 am
content: Well I finally got a riser card from a Packard Bell system (Model KJ012404 P/N: 146230) that has 2 PCI slots and 3 ISA slots and I was able to make it work in the AST motherboard by modifying it. I had the same experience as the first poster where the system would not boot with the unmodified riser in it. I finally figured out that because this card has the 3.3V for the PCB bus wired up to some of the used pins on the AST motherboard it would short out the address bus pins AD19 and AD20 in pin location A 23 and B23 and this is why it would not boot. I covered up the pins with tape as they were already duplicated on pin A 21 for AD19 and pin B20 for AD20 and the system then did boot. However it showed an error in device manager about not being able to assign a IRQ to the card with a Code 29. I finally traced this down to the IDSEL (Init Device Select) pin on the PCI slot being wired on the riser to the incorrect address line resulting in the BIOS being unable to see the card to configure it as the configuration address space was in the wrong location. I finally figured out that on the riser card for the top PCI slot (J2) had IDSEL wired to AD28 and the lower PCI slot (J5) had IDSEL wired to AD30. As these traces were either under the PCI slots or inside the layers of the PCB I decided that the best option of disconnecting them was to fry them with my power supply set at 5V and about 5A of current. I then connected the top PCI slot to AD20 and the lower PCI slot to AD19 and then the error message in the device manager went away and I was able to use both PCI slots. I think the orignal riser for this motherboard would have used the pins at locaton 23A and B for setting the IDSEL for the PCI slots and that is why you can not place a generic PCI ISA riser card into this motherboard. I hope this helps anyone who wants to make a riser card for this system.