VOGONS


First post, by retroegde

User metadata
Rank Newbie
Rank
Newbie

Without getting into serious details I'm wondering if anyone has information on a MFM / Floppy disk combination card that is 16-bit ISA but will work on an 8-bit bus. I have the ability to load a BIOS on the Intel networking card I have installed in my 5150 PC.

Googling found a forum post of someone getting it to work but the URL is no longer valid
https://vcfed.org/forum/showthread.php?48479- … ebay-components

Searching archive.org only has the first page of the thread.

Searching stason.org shows a handful of 8-bit cards but I don't know how rare they are.

Reply 1 of 2, by jakethompson1

User metadata
Rank Oldbie
Rank
Oldbie

If they were talking about it working, they were probably talking about letting the extra half of a 16-bit I/O card overhang, and only use the floppy/serial/parallel part disregarding the hard drive. Only the IDE hard drive part is actually 16 bits, so that works and is easier to find than an 8-bit I/O card with 1.44MB floppy support.

By "MFM" if you mean a WD1003-compatible card (and IDE is software-compatible with it) you can't use it in an 8-bit slot. It expects the processor to use REP INSW and REP OUTSW to communicate with it, i.e. I/O transactions with a full 16 bits at a time. It won't be able to accept 8-bit I/O and reconstruct two of them into one.

An 8-bit MFM card is different; it tends to use DMA channel 3 and IRQ 5. 8088 processors are so slow (and lack i/o string instructions) that ISA DMA is faster than programmed I/O so that is why it was used. Not to mention, another problem you would have with a 16-bit card is that they are hardwired to IRQ 14 which you don't have. So in other words the circuitry on an 8-bit MFM hard drive card is completely different than a 16-bit one and it can't just downgrade itself.