5.25 floppies that read but don't write

Marc Verdiell marc.verdiell at gmail.com
Fri Aug 21 23:50:55 CDT 2015

I looked at the pin 2 signal and resulting drive behavior. It doesn't
properly auto-switch between densities as it should. It does switch it for
reading, but not for writing. Below is the full story.

There is one jumper that controls DD/HD switching mode. 

In the auto position, I can read both HD and DD formats (I use the right
diskettes for either format). You can see the computer on pin 2 (density
select) trying both positions, settling on the right one (high for HD, low
for DD), and reading the disc correctly.
That's great. But when writing, although pin 2 goes to the right level, the
floppy seems to ignore it. It will always write at DD, and refuse to write
in HD (no signal at the head).

Interestingly, in the fixed density position (should be HD only), it WILL
write at 1.2M HD! The write signal does appears at the head. Unfortunately
in this setting it will NOT switch the reading, get stuck reading only DD
and fail on HD. 

So I can either have the reading HD or the writing HD, but not the two at
the same time. That's why a regular format fails. The density switching
logic seems to have a problem. Rather than track down which IC or transistor
failed on the board it might be much simpler to get another 5.25 floppy. In
the meantime my understanding of these simple critters has improved a lot.


>At high density, have you taken a good look at pin 2 of the floppy 
>interface?  Have you checked to see if pin 2 is configured (via jumpers) 
>as "density select"?  

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