From: Spiro Trikaliotis (ml-cbmhackers_at_trikaliotis.net)
Date: 2006-05-13 18:13:20
Hello,
a follow-up to my own mail (quoting again). I think others might find
this very interesting, too:
---- snipp -----
I think, I found the true originating source to Garth Wilson's
explanations. It is the 6522 datasheet from Synertek, who knew
in 1982 (!!!) from _several_ 6522 shift register bugs and
corrected one of them. Unfortunately they did not correct _the_
bug.
Read more in:
http://www.6502.org/documents/datasheets/synertek/synertek_sy6522.pdf
page 7 (8 in the PDF), section 5.1, "Shift Register Warnings".
Wow, it _only_ needed 20 years to find out, why _really_ the
1540/41 disk drive is/was so damn slow ;-) Commodore really had
some bad luck with the 1540 design. They could have easily fixed
the bug, if they knew Synerteks datasheet <sigh>.
Garth Wilson's adds fundamental exaplanations by pointing out
that this behaviour only affects teh shift register, if it is
clocked by an external clock at CB1 asynchronous to Phi_2.
Soooo, the 1541 drive and other non-CIA equipped drives _should_
be able to send and receive data with Commodores Serial Burst
protocol as long as the floppy disk drive (C64/C128) always
controls the clock, for reading and for writing.
But since you also need to do some modifications, you could add
that D-FlipFlop circuit with not much higher cost.
---- snipp -----
Note: This is an application note of Synertek, not a data sheet.
Regards,
Spiro.
--
Spiro R. Trikaliotis
http://www.trikaliotis.net/
http://cbm4win.sf.net/
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