Re: Two 1541-II drives with problems

From: Gerrit Heitsch <>
Date: Tue, 22 Apr 2014 19:14:09 +0200
Message-ID: <>
On 04/22/2014 06:49 PM, Ville Laustela wrote:
> Hi.
> Thanks Spiro, that program sounds like a great idea.
> I made a test over ZoomFloppy (from WinVICE) and the drive 1 of the two drives.
> I typed in the program and run it and the first run gives "73,CBM DOS V2.6 1541,00,00" and the following runs give "00, OK,00,00". So it appears that the fault must lie at the DC VIA or the S.O bit on the 6502 (can I check this with a logic probe?).

Well, the SO-Bit needs to change state when the drive is running and the 
DOS has enabled that functionality through CA2 of the VIA that controls 
the drive. It is supposed to indicate that a Byte is ready to be read. 
That doesn't mean that the 6502 is reacting to it though, that part 
happens inside the CPU where you cannot reach with a logic probe...

You should grab the schematics for the 1541 long board. That one doesn't 
have the gate array, it does everything in discrete TTL which makes it 
much easier to find out HOW it's done.


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Received on 2014-04-22 18:00:03

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