AT&T 3B2 floppy format

Fred Cisin cisin at
Sat Apr 2 19:37:28 CDT 2016

On Sat, 2 Apr 2016, Seth Morabito wrote:
> After doing more experimentation this morning, I've discovered that
> the data mode I should have been using was 250kbps MFM, NOT 300kbps!
> After changing ImageDisk to write 250kbps (and ensuring that the
> 300RPM "I" jumper was strapped on the FD55-GV), the images work
> perfectly!
> So, that's the secret. 3B2 floppies are DSQD 250kbps MFM.

That is an issue of whether your drive is turning at 300ROPM, or at 
If you use a 55F drive, or a Tandon TM100-4, etc. such as was used on the 
original machine, then the drive turns at 300 RPM, and the data transfer 
rate is 250Kbps.

OTOH, 1.2M drives trurn at 360RPM.  (at loeast while doing 1.2M)
SOME are dual speed, and can/will switch to 300 RPM, to use the normal 
250Kbps data transfer rate, when doing something o0ther than 1.2M.
Others keep turning at 360RPM, and expect you to correct for that by using 
a 300Kbps data transfer rate.

Grumpy Ol' Fred     		cisin at

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