FC5025 USB 5.25 floppy controller - first report

[Steven Hirsch]

On Tue, 23 Feb 2010, Tony Duell wrote:

>>> Pullups on late 5.25" (e.g. 1.2MB) floppies as well as 3.5" drives
>>> tend to be in the 2K range.  Pullups on an 8" drive are almost always
>>> 150 ohms.  How much current can that microcontroller sink for
>>> sustained periods?
>>
>> Hm.  Never realized that.  However, since the pullups are generally DIP
>> resistor packs why not replace them with a higher value?
>
> Well, you could, but I wouldn't. The resistors are not only pull-ups,
> they also terminate the cable (regarded as a transmission line). I am a
> great beleiveer in properly terminated cables, reflections on said cables
> cause all sorts of 'interesting' problems.
>
> I suspect it would work with 1k pull-ups and a short cable, but
> constructing a buffer board is not going to take long (it's only 2 or 3
> 14 pin TTL chips), and it means it _will_ work. If you have problems with
> data corruption, etc, you then know it's not due to ringing on the cable.

Fair enough :-).  The TTL chips of which you speak are bidi bus 
transceivers, correct?  Would you use the R/W line to switch directions, 
or is there other magic involved?

Any construction tips?  (Schematic would be nice - but hand-waving works, 
too!)

Steve


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