Acorn System Refurbishment (Floppy Disk Controller)

 

I've wired up a Floppy Disk Controller following the original Acorn schematic:

 

It works with both 3.5" and 5.25" drives double sided and 40 or 80 track.

I have managed to get old PC drives working by changing the links to set the rotation speed to 300rpm (rather than 360rpm).

To get 3.5" drives working a simple mod is required to overcome the compatibility issue with the 8271 controller IC.  Most 3.5" drives drop the Index pulse when the head is stepped this results in the Ready signal dropping out and a drive not ready error.  The clock chain that generates the timing signal for the Ready signal ends in a CD4020 which normally generates a 1.9Hz clock for the CD4013 latches.  By good fortune there is an extra stage unused on the CD4020 which can be used to generate a 0.95Hz clock instead, this stops the Ready circuit timing out when a single Index pulse is missing (this mod should work on Atom and BBC's fitted with the 8271 as the interface circuit is the same).  All that is needed is to move the clock from pin 2 of the CD4020 to pin 3.

I have only been able to test with three types of 3.5" drive: Sony MPF920 (CXA8061 and BH955 versions) and Mitsumi D359M3D.

3.5" drives seem to all be set to drive 1 and don't usually have user accessible links to change drive select.  The MPF920 version with the BH955 IC can be changed by moving a zero ohm link from position JC31 to JC30, this requires careful soldering !  The CXA8061 version doesn't have these links and requires the addition of a wire link and a track cut as shown in this photo.

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