I like the idea 😀
I usually need only one ISA slots if I use DOS, for sound. Sometimes a second one of u want to test/program an XT CF card. But if I use just Windows 9x or higher, ISA does not have any real benefit to me

Especially with the ROM and 3,3v extension it can be a nice upgrade. Lots of PCI cards are just half height (Lan, modem, ATA). The hard part would be to get the height exactly.
Bonus would be to have extenders to screw the daughter board directly onto the base, on too of the existing screws.
It would be even more flexible if you can choose between 1, 2 or 3 PCI slots, with variable board size. Lots of boards have only one or two ISA slots to cover, especially uATX ones.
The daughter board would need some additional logic to split the PCI... But that's existing tech 😀

waterbeesje wrote on 2022-04-27, 09:39:

I like the idea :) I usually need only one ISA slots if I use DOS, for sound. Sometimes a second one of u want to test/program a […]
Show full quote

I like the idea 😀
I usually need only one ISA slots if I use DOS, for sound. Sometimes a second one of u want to test/program an XT CF card. But if I use just Windows 9x or higher, ISA does not have any real benefit to me

Especially with the ROM and 3,3v extension it can be a nice upgrade. Lots of PCI cards are just half height (Lan, modem, ATA). The hard part would be to get the height exactly.
Bonus would be to have extenders to screw the daughter board directly onto the base, on too of the existing screws.
It would be even more flexible if you can choose between 1, 2 or 3 PCI slots, with variable board size. Lots of boards have only one or two ISA slots to cover, especially uATX ones.
The daughter board would need some additional logic to split the PCI... But that's existing tech 😀

I know that with two aditional slots that a basically passive solution is possible with nothing more than maybe a few configuration jumpers. Wether or not 3 or more requires a controller, I’m unsure.

But I have a two pci slot riser that works with no additional logic.

Using standoffs to mount it is one way, but I think only two are in convenient locations, others are over where there may be cables and board components such as a processor on AT

I was thinking it could be edge connectors slotted into the isa slots that could mount the card. Soldered into slots on the riser. Given several, It could be a significant holding force.

I was thinking it may actually even be possible to have a slot at the tap location also, but that may get tricky. I know there are edge mounted pci slots, perhaps this could help somehow, maybe it could attach to the riser via a right angle pin header breakout

Alpha 1 just sketching this out, it'll get better.

I'm thinking that the top most pci can just be a reverse pci on the back (gotta flip footprint) joined by a double sided pci straight thru piece.

Still unsure if this will be a active or a passive device. More research needed. I know that a passive 2 slot device is possible though. but 4 slots? unsure. more research needed. something tells me that 4 slots will need active management. Though I may turn it into a 3 slot unit or make several flavors with different numbers of slots. Will probably add a sata power plug to it.

Not all signals are connected in parallel for PCI slots. It is only true for ISA Slots. For example not all IRQ lines are available on single PCI Slot.
Note that in some BIOSes, you can assign an IRQ for the corresponding PCI slot.

zyga64 wrote on 2022-06-21, 10:11:

Not all signals are connected in parallel for PCI slots. It is only true for ISA Slots. For example not all IRQ lines are available on single PCI Slot.
Note that in some BIOSes, you can assign an IRQ for the corresponding PCI slot.

somehow they made this work. ( I have one) it’s buried in my stuff atm but looking at the pic now I see a chip that I didn’t remember before.

Think I can pull IRQs from the ISA slots? gonna be plugging into them anyway...

It's not just IRQs, each slot also has individual busmaster control lines to the chipset

I think this interface may work only with dedicated motherboard in dedicated PCI slot.
While "normal" cards works in this slot "normally", special cards may benefit from its "special" features.

In the Socket A (462) era, there were Medion motherboards (manufactured by MSI) with one PCI slot in blue.
Special combo cards (such as Medion 7134 DVB-T/modem) work fully only in this slot.

I have no idea about PCI/ISA IRQ compatibility. I'd rather assume it won't work.

So there is this thing, which admittingly is a buggy card, but it seems to have no less than three pci devices on one card. Without a multiplexing chip? Any thoughts as to what’s going on here?

Sphere478 wrote on 2022-06-21, 10:32:

zyga64 wrote on 2022-06-21, 10:11:

Not all signals are connected in parallel for PCI slots. It is only true for ISA Slots. For example not all IRQ lines are available on single PCI Slot.
Note that in some BIOSes, you can assign an IRQ for the corresponding PCI slot.

somehow they made this work. ( I have one) it’s buried in my stuff atm but looking at the pic now I see a chip that I didn’t remember before.

Think I can pull IRQs from the ISA slots? gonna be plugging into them anyway...

That chip you didn't remember before may have been a PCI-PCI bridge.

Interrupt lines are not the problem. Each PCI slot has 4 interrupt pins, INTA# through INTD#. PCI interrupts are designed to be shared; with 5 or 6 PCI slots and 1 AGP slot (which just a fancy variant of PCI after all) on an ATX motherboard plus any PCI devices soldered onto the board plus the PCI devices integrated into the southbridge, there's far more devices than interrupt lines. In practice, motherboard makers would rotate the PCI interrupt lines coming from the southbridge, so that INTA# did not get overused by PCI cards that only needed 1 IRQ. So slot 1 would have INTA#, INTB#, INTC#, INTD#, and then slot 2 would have INTB#, INTC#, INTD#, INTA#, and so on.

As for the ISA IRQs, they behave entirely differently and are edge triggered instead of level triggered IIRC, and the PCI Interrupt Controller will not be expecting PCI devices to use them. (PC/PCI aka SBLink is a separate thing from this.)

There are several signals on each PCI slot that are not a "bus", they are unique per-slot. From Wikipedia:

Most lines are connected to each slot in parallel. The exceptions are: […]
Show full quote

Most lines are connected to each slot in parallel. The exceptions are:

• Each slot has its own REQ# output to, and GNT# input from the motherboard arbiter.
• Each slot has its own IDSEL line, usually connected to a specific AD line.
• TDO is daisy-chained to the following slot's TDI. Cards without JTAG support must connect TDI to TDO so as not to break the chain.
• PRSNT1# and PRSNT2# for each slot have their own pull-up resistors on the motherboard. The motherboard may (but does not have to) sense these pins to determine the presence of PCI cards and their power requirements.

The REQ/GNT lines are bus mastering. The IDSEL is a slot identifier. I think these signals are why PCI-PCI bridges are a thing - so that bus mastering and slot identification can be shared.

Good infos 😀 thanks!