Re: 8520

From: Ethan Dicks (
Date: 2000-08-02 01:23:55

--- Richard Atkinson <> wrote:
> On Tue, 1 Aug 100 wrote:
> > > 8520 has clear differences in the software model (no TOD clock) and does
> > > not concern us here.
> > 
> > A friend of mine succeeded in ataching several PC 720KB-drives to 1581. The
> > 8520 is the only IC in the design you cannot buy in a shop. So for this
> > reason we are interested:
> > - in what way a 8520 differs from a 6526

I think the 8520 can be hooked to a faster bus (it's used in the Amiga, after
all).  The TOD clocks are binary timers.  There may be some other subtle
differences of which I am unaware.

> Sorry, Ruud, I forgot about the 1581. You're absolutely right.
> Speaking of which, can either the 1581 or CMD FD-2000 use more than one
> disk on the shugart bus? Apparently it was specified for up to four
> devices, but IBM only ever used two.

The standard 34-pin floppy arrangement allows for 4 drives, but not all drives
support the older drive select arrangement.  Some newer drives only listen
on twisty cables as drive 1.

This doesn't mean that it can't be worked around, but even if the 1581
circuit _could_ select 0-3, not all drives can be so selected without an
external circuit.

I'd be pretty happy with a dual 1581 myself. I happen to have a drive or
two lying around (one has a misaligned mechanism and only reads/writes
its own floppies).  I may give that a go.


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