RE: Commodore "Most Wanted" List

From: Baltissen, GJPAA (Ruud) <>
Date: Thu, 3 May 2012 08:19:14 +0200
Message-ID: <>
Hallo Didier,

> Double	 PC/XT	 9	 80	 2	 720 Kio	 250
> Haute	 AT	 18	 80	 2	 1 440 Kio	 500 kbit/s	
> I dont remember the exact details but the speed of the data is
> probably twice

9 sectors * 512 bytes/sector * 6 rev/sec * 8 bits = 221184 bits/sec
IMHO the number is even a bit too low.

221184 bits/sec = 27648 bytes/sec = 36 uS/byte which isn't a suprise.
Although a 720 KB is roughly four times a 1514 (after GCR), it also has
twice the number of sides and tracks. These numbers tell us that even a
1 MHz 6502 can handle a 720 KB drive. More interesting: using the same
argument, a 2 MHz 6502 should be able to handle a 1.44 MB drive. 

Why didn't I find this out myself before? Because I did those
calculation before but forgot about the two sides and got numbers twice
as they should be :( 

AFAIK it is just a matter of swapping two wires so one can use a PC 720
KB drive on a 1581. Never having had the need for it, I never tried it
out myself. Being able to connect a 720 KB drive, a 1.44 MB one
shouldn't be a problem either. 
Next thing: what about the density? The 1.44 FDD has a switch inside
that tells the controller whether a 720 KB or a 1.44 MB floppy has been
inserted. If I am correct, pin 2 of the Shugart bus is used to give the
FDC this information. In the 1581 this pin is connected to the 8520 so
it can be read. 

So are there more bears on the road that could stop us to connect a 1.44
MB FDD to a 1581 and use it?

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