WTD: 5.25' DSDD drive & Acorn Master128 keystrip scan/schematic

[Tony Duell]

> 
> >   I'm after a 5.25" 80-track (preferably 40/80 switchable) DSDD drive,
> 
> Never heard of that, and I can't seen any way it could possibly work.
> 5.25" drives either have a wide head gap for 48 TPI (35-track or
> 40-track), or a narrow head gap for 96 TPI (80-track) or 100 TPI
> (77 track).  You can't "switch" without replacing the heads.

Such drives were common on BBC micros in the UK.

They have the 96 TPI head, but with a hardware double-stepping circuit 
added to the drive logic. THey will therefore reliably read and write 80 
cylinder disks and read 40 cylinder ones. Writing 40 cylinder disks has 
all the usual problems.

The standard Acorn DFS (Disk Filing System) didn't support 
double-stepping in software, so this was one way to be ablle to use both 
types of disk in the same drive.

I seem to remeber seeing a similar circuit for one of the TRS-80 machines 
either in 80 micro or Rainbow. 

-tony