Caluser2000 wrote on 2021-09-17, 00:40:
After the 32-Bit EISA bus was defined by the Gang of Nine,
the specs for the previous 16/24-Bit AT bus were set in stone as a side effect.
That was necessary, because EISA had the old slot contacts as a legacy built-in.
The resulting "ISA" bus was similar to the AT bus, but not exactly the same.
ISA describes the timings for the bus, including a reference frequency of 8.33 MHz.
AT bus differed from this. The original PC AT Model 5170 ran at 6 MHz, originally, and had no upper limit yet (open ended, if you will).
(That's why some late 80s expansion boards were specced for up to 12MHz.)
According to the later ISA specs, that 6MHz PC AT must have run its bus out-of-spec, thus - if it was considered an implementation of ISA, which it wasn't.
To be fair, though, later ATs increased operation frequency to 8MHz, which still was a bit below 8.33 MHz.
However, that's not all, bus cycles might have been different, not sure.
You know, rising and falling edges. That low-level stuff.
To be honest, I'm too lazy to check right now.
Anyway, these are merely details.
It's comparable to AT-Bus vs IDE vs ATA0, I believe.
Most people will make no distinction between these.
To them, AT bus equals ISA equals and PC bus equals XT bus equals 8-Bit "ISA".
Which in most cases is close enough.
But there are subtle differences sometimes.
The original PC 5150 had one slot that was special, I vaguely remember.
Anyway, I just think that's interesting. 🙂
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