Simple mod for 3.5" Floppy Drives on an 8271 ?

[ChrisO]

Having got 5.25" PC drives working on my System computer the next thing to try is a 3.5" drive. There is of course a problem when using these drives with an 8271 controller. Anyway I plugged a Sony MPF920 drive in to the Beeb and it works with small files but as soon as you try saving anything bigger than a few hundred bytes I get the expected Drive not ready error.

A quick look with a scope confirms that the index pulses are missing when the head steps.

There are fixes around for the Beeb - modified drives from sprow and/or patched DFS from JGH but is it not possible to just extend the timeout in the Ready circuit ?

The final 4020 in the clock chain has an extra stage unused that could be used to provide a 0.95Hz clock instead of the 1.9Hz clock, would this work ?

I didn't fancy modifying my Beeb so I've moved the drive to the System computer and modified that instead (moving the clock from pin 2 to 3) and it seems to work but am I missing something, does a mod this simple really do the trick ?

Chris

[danielj]

Hi Chris - I think Rob Sprowson has the best description of this:

http://www.sprow.co.uk/bbc/floppydrives.htm

d.

[ChrisO]

Yes I have looked at Sprow's page, his solution is to modify the drive by adding a Microchip PIC to reinsert the 'missing' index pulses, an alternative approach is JG Harston's patched DFS 1.21 which essentially ignores the drive not ready error. I thought I'd try fixing the underlying problem which is the 8271 interface. Its a pretty simple mod and seems to work but I wanted to make sure I hadn't missed something ? perhaps there are performance issues with extending the timeout ??

Chris

[CMcDougall]

sure someone said the Sprows mod drives still don't work anyways

the JGH 1.21 dfs works to a fashion, but as you say after using long files to load /save it then gives bad errors & even unformatted discs but put back in a 1770 beeb and all is well again.

One of my 8271 beebs has the switches on right bottom of keyboard, they just made the drive less noisy & load quicker, but still gave errs after a while.

so to conclude 8271 = 51/4" drives only, as back then 1971, no 31/2" drives where invented...