12 May
2008
12 May
'08
8:06 a.m.
Allison wrote:
It would not be diskless only floppy less.
Er, I don't what to get into the subject of what a "disk" is, but
I'll concede that the beast would have secondary storage.
IF you really want to enjoy the vintage experience you
can include a
floppy controller but be warned...They are a PAIN to use and program.
The most important detail is the unless you include DMA (more parts)
the cpu does all the heavy lifiting in real time and that requires
tight code or some hardware tricks (more parts). It stops getting
simple real fast. Then there are the various floppies with their
interface quirks.
I disagree--I've been known to program a floppy controller or two in
my time and never found them particularly nasty--except for the
WD1781 which had a really annoying tendency to hang during some
operations. WD never fixed that--the 'B' part still had the problem
when they discontinued it. Our fix involved timing the operation and
then resetting the controller if it went out to lunch. So when the
PC came along when a drive-not-ready condition would cause the 8272
to hang, it seemed like old home week.
(Does anyone have a datasheet for the WD1781? I can't seem to locate
my copy anymore.)
A WD1770 is very easy program (as are most of the WD x7xx parts),
requires very little in the way of interface logic and can be
serviced with non-DMA data transfers, even on a 2MHz 8080. (Herb
Johnson has a good section on floppy transfer vs. CPU speed on his
retrotechnology.com site.) A 4MHz can easily handle DD 8" drives
without DMA, which should mean that HD 1.44MB drives should also
work. Don Tarbell's first 8" controller handled SD 8" on a 2MHz 8080
quite reliably.
The topic interested me at one time because I wondered if it was
possible to read DD 8" floppies without DMA with a 2MHz 8080. I
think it is, but you have to resort to some strange programming
tricks. I believe that Herb has my notes on this.
Cheers,
Chuck