@Marco
I think we switched the topic to IDE controllers now, right ?
The same PC from above but with Promise EIDE Pro.
DTK PT-227 on another, most pimped, 386 machine.
SpeedSys reports 486 class CPU, but it is confused.
Both tested with CF cards. Kind of touching the possible upper bound of ISA/IDE/PIO.
Latest models IDE mechanical HDDs with cache and everything can saturate these puny old ISA controllers very well, but there is a lot of volatility with mech HDDs, so i stopped testing with them.
In reality, good quality present day CF cards are much faster than old HDDs just because of seek time.
To your question.
Partially answered with the mech HDDs note, but also - things don't scale linearly.
Towards the upper end more and more MHz'es are needed for smaller and smaller improvement.
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@douglar
If your question was towards me - no proprietary software drivers needed for all ISA SCSI controllers i touched so far, including the AHA-1542 noted above.
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Maybe i should start a thread for ISA IDE similar to the VLB IDE one.
Have quite a bit of info that can be dumped for public consumption.