Re: The strange design of the 1551 floppy drive

From: Spiro Trikaliotis <ml-cbmhackers_at_trikaliotis.net>
Date: Sat, 12 Nov 2011 21:05:52 +0100
Message-ID: <20111112200552.GX30372@trikaliotis.net>
Hello,

not knowing ANYTHING about the 1551:

* On Sat, Nov 12, 2011 at 04:48:57PM +0100 Gerrit Heitsch wrote:
 
> - The I/O-port (full 8 Bit) in the CPU is used for drive control.
>   There is no SO-pin, suggesting a very different approach to
>   data transfer between the drive hardware and the controller.
>   Maybe that's the reason for 2 MHz CPU clock?

I would expect it to be the other way around: The 1570/1571 has a SO on
the 6502, but it is used in 1541 mode (1 MHz) only. As soon as the
1570/1571 specific mode is used (2 MHz), the SO is not used; instead,
the VIA port is polled.

I assume that the 2 MHz allows this polling without fear to be too late,
while with 1 MHz, this would not be reliable.

But, I must admit, I did not do any calculations (yet) to check this
assumption.


Regards,
Spiro.

-- 
Spiro R. Trikaliotis                              http://opencbm.sf.net/
http://www.trikaliotis.net/                     http://www.viceteam.org/

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Received on 2011-11-12 21:00:12

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