5.25 floppies that read but don't write

Marc Verdiell marc.verdiell at gmail.com
Thu Aug 20 15:04:54 CDT 2015

Following Chuck's advice, I scoped out the pin 22 and 24 (write data and
write gate) on the floppy, and they looked fine. Then being curious, I
managed to figure out what the write wire for Head 0 was. And I discovered I
got nice matching writing pulses at 300 kHz (formatting at 360k density),
but none at 500 kHz (1.2 Mb density). Results posted here:
So I tried to force formatting in DOS at 360k, and sure enough it worked! I
can then read the diskette back, write on it, etc... And of course it failed
formatting at 1.2 Mb. But the drive (Chinon FR-506) is a 1.2M one, and reads
fine at 1.2M! Any clue? Is there a drive setting that would prevent it to
write at high density but let it do at low density?


> Date: Tue, 18 Aug 2015 21:30:31 -0700
> From: cclist at sydex.com
> To: cctalk at classiccmp.org
> Subject: Re: 5.25 floppies that read but don't write
> On 08/18/2015 09:05 PM, Marc Verdiell wrote:
> >
> >
> > I have connected a 1.2M 5.25" floppy to my computer. After a bit of
> > jumper learning and setting, it's recognized and reads my old DD and
> > HD floppies fine. But for the life of me I cannot write to it. Not
> > under DOS, Win98, or WindowsXP. Which all read fine.
> Make sure that "WRITE GATE" goes low (use a logic probe) on pin 24 of 
> the drive while you do your write.  If so, your floppy write circuitry 
> has a problem.  I.e., it's not the cable.
> --Chuck

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