The strange design of the 1551 floppy drive

From: Gerrit Heitsch <>
Date: Sat, 12 Nov 2011 16:48:57 +0100
Message-ID: <>

thanks to a reader of this list I now own a 1551 floppy drive for my 264 
systems. I always liked the 1551 since it has a few nice features 
besides the parallel cable. Formatting a disk is a lot faster than with 
a 1541 and the spindle starts up for a few seconds when inserting a disk.

On the other hand, this drive seems to be a complete departure from all 
the other single drive floppies like the 1541/1571/2031 that
use a 6502 with 2 6522:

- The CPU is a 6510T running at 2 MHz (measured by me, most pages
   on the web claim 1 MHz).

- 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?

- All other I/O is done with a 6525A (as was used in the CBM600/700)

- There are no timers in the 6525A so it cannot produce a periodic IRQ.
   To get around this, a 555 is wired as an oscillator, producing a
   short pulse for the IRQ-pin of the 6510T about every 10 msec.
   Since the 555 is not controlled by a crystal, the timing can't be
   very precise. This is the only IRQ source, meaning all I/O
   would have to use polling.

- The interface that plugs into the 264 uses a 6523T/6523A-28
   as the I/O chip (seems to be a 6525A in 28pin, there is a
   manual how to replace a blown 6523T with a 6525A) and a
   PLA 251641-03. This interface is unable to produce IRQs,
   looks like polling on the host side as well.

- There seems to be only one ROM revision (318008-01) that appears
   to have no 1551-specific bugs (at least I don't know of any.
   Anyone know whether it has the SAVE@-bug?). What genius was
   able to take this new architecture and get the -01 ROM working?
   The ROM has this string:

- The large gate array has no Commodore part number, it's done by
   Hitachi and marked HD61J215P. Is it the same as the one used
   in the 1571 or one specifically made for the 1551?

All in all, this suggests that this drive uses at least 3 maybe 4 chips 
that were specifically made for it plus the ROM containing the software. 
Even with MOS next door that appears to be a rather expensive design.

Does anyone here have some information why it was done this way? I mean, 
Commodore could have done the same with a slightly modified 1541 design 
(one of the VIAs has that unused 8 Bit port in the 1541), that should 
have been a lot cheaper on the hardware side, at least for the 
controller board in the drive. After all, other parts were taken from 
the 1541C design (the hybrid and 20pin Gate array 251829-01)


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Received on 2011-11-12 16:00:03

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