FC5025 USB 5.25 floppy controller - first report

[Steven Hirsch]

On Sun, 21 Feb 2010, Chuck Guzis wrote:

> On 21 Feb 2010 at 10:01, Steven Hirsch wrote:
>
>
>> Why would a Shugart 801 represent a different load than a 5.25" drive?
>> Aren't they both 5V TTL level interfaces?
>
> 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?

> You might get away with it, but I wouldn't recommend it.  Add to the
> mix that 8" drive cables are usually much longer than 5.25" or 3.5"
> cables (read:  higher inductive load) and you have an "interesting"
> cocktail.  I never could make my CW MK 3 work reliably with 8" drives
> until I constructed a buffer board.

Even more interesting.  I've noticed that my CW MKIV+ is "fussy" about the 
drives it works with.  Now I have an explanation!  Before going to the 
trouble of building buffers, I'm more inclined to use a higher value 
pullup and a shorter cable.

Steve


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