Max no. of 16 bit ISA slots in a 386 board \ VOGONS

Max no. of 16 bit ISA slots in a 386 board

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First post, by Jackal1983

Posted on 2022-04-12, 15:07

Jackal1983 Offline

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So I've noticed that it seems most of the older ISA boards i've found typically have a couple of slots with unpopulated 16 bit isa slot extensions. My Peak D/M based board has a total of 8 slots but only 6 of them are fully 16 bit. Is this just a cost saving measure, or is there a more solid limitation (e.g. DMA/IRQ address limitations). I was planning on swapping one of the slots around ( the retail version of the board has the 8 bit slots at slot 1 and 7 instead of slot 1 and 2 like mine) but if I can just add the unpopulated slots safely and get a full 8 16 bit slots I will.

Reply 1 of 5, by rmay635703

Posted on 2022-04-12, 15:13

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There are certainly resource limitations but normal motherboards offered both 8 and 16 but slots because some cards would only physically fit in an 8 bit slot

Also many cards remained 8 bit throughout the lifespan of the device (like modems and serial)

I have never seen a specific limit on how many slots are allowed but have found boards with 16 expansion slots, not sure if there is an electrical limit or not

https://globalamericaninc.com/product/hbp-12s … sive-backplane/

Reply 2 of 5, by maxtherabbit

Posted on 2022-04-12, 15:14

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some very old 8-bit cards actually extended down below the edge connector behind the 8 bit slot, omitting the 16bit extension was a mechanical backwards compatibility measure

Reply 3 of 5, by Jackal1983

Posted on 2022-04-12, 15:19

Jackal1983 Offline

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maxtherabbit wrote on 2022-04-12, 15:14:

some very old 8-bit cards actually extended down below the edge connector behind the 8 bit slot, omitting the 16bit extension was a mechanical backwards compatibility measure

Yeah, I didn't think about that. I've seen a few cards like that, the original XT floppy controller for example.

Reply 4 of 5, by Babasha

Posted on 2022-04-12, 15:28

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Here list of 386 motherboards in TH99 database. Many boards have 7-8 ISA16 slots.
http://www.uncreativelabs.de/th99/m/m386_i.htm

Need help? Begin with photo and model of your hardware 😉

Reply 5 of 5, by mkarcher

Posted on 2022-04-12, 16:18

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Jackal1983 wrote on 2022-04-12, 15:19:

maxtherabbit wrote on 2022-04-12, 15:14:

some very old 8-bit cards actually extended down below the edge connector behind the 8 bit slot, omitting the 16bit extension was a mechanical backwards compatibility measure

Yeah, I didn't think about that. I've seen a few cards like that, the original XT floppy controller for example.

A lot of the original XT cards did it. The original CGA is another one. I was going to also mention the MDA, but I double-checked: The MDA doesn't extend down behind the ISA connector.

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