Re: Patching in a CBM disk drive connection in a C64 case.

From: Rhialto <>
Date: Sun, 3 Apr 2022 14:30:19 +0200
Message-ID: <>
On Fri 01 Apr 2022 at 14:30:13 -0500, Jim Brain wrote:
> (question $07b)

from that: "We early PET/CBM freaks knew, from playing music, that there
was something wrong with the 6522's shift register:  it interfered with
other functions.  The rule was:  turn off the music before you start the
tape!  (The shift register was a popular sound generator)."

Do we already know why the shift register interfered with the tape?

and, to remind people, do we already know why pressing the < button
while loading / scanning the tape for something to load would print
< characters to the screen if there was a signal on the tape?

Neither are emulated in VICE (I think I tried it once) so it isn't
something that trivially occurs...

Also, Compute #3, march/april 1980, mentions that sound
doesn't work if you have C2N #2 plugged in. Page 88:


Larry Isaacs

Some of you may have been unable to get the CB2 Sound, described in last
month's RAMBLIN', to function. Fortunately the most likely cause has a
very simple solution. Unplug the external cassette from the rear of your
PET. Having an idle cassette plugged into that cassette port grounds the
CB1 line on the 6522 IC in the PET. Grounding CB1 unfortunately stops
the shifting of the shift register used to generate the CB2 Sound. I
also checked a Synertek 6522 (PETs use MOS Technology) and found that it
had the same problem.  It appears that this is inherent in the design of
the 6522, so keep it in mind when using the shift register for output.

___ "Buying carbon credits is a bit like a serial killer paying someone else to
\X/  have kids to make his activity cost neutral." -The BOFH    falu.nl_at_rhialto

Received on 2022-04-03 15:00:07

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