Re: 8520

From: Nicolas Welte (
Date: 2000-08-02 10:28:59

Hallo Ruud, wrote:
> 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

As Richard said, the 8520 has a straight 24bit binary event counter instead
of the 6526 TOD clock.

> - does the software use the difference

Yes, the event counter is used to read the state of a jumper on the 1581
board. This jumper is either connected to Phi2 or not, so the counter is
either running or not. Depending on the result the ROM code assumes that a
WD1770 or WD1772 is present. The two chips have different stepping rates
and have to be configured properly.

> - can the software be altered to use an 6526(A)

Of course.

But in my opinion the 6526 is a bad choice, it is even more obsolete than
the WD1772 and even harder to get, and also not manufactured anymore. The
8520, on the other hand, is a common Amiga component and still available
from many Amiga surplus stores. You can even have it in a PLCC variant that
saves some board space. Just make sure you get the 8520A (2MHz version).

BTW, the 1570/1571 drive can use any 2MHz CIA, Commodore put in whatever
CIA they had. I was already thinking about replacing some 6526As in there
by 8520As just to get a few working spare 6526A chips for the computers.

This message was sent through the cbm-hackers mailing list.
To unsubscribe: echo unsubscribe | mail

Archive generated by hypermail 2.1.1.