FC5025 USB 5.25 floppy controller - first report

Tony Duell ard at p850ug1.demon.co.uk
Mon Feb 22 14:33:35 CST 2010

> Why would a Shugart 801 represent a different load than a 5.25" drive? 
> Aren't they both 5V TTL level interfaces?

A Shigart 801 would be the same load as a 'classic' 5.25" drive, but not 
the same as a modern-ish half-height ex-PC drive.

The difference is the termination/pull-up resistors. The original spec 
(for 8" -- SUhgart SA800 series, etc, 5.25" -- Shugaert SA400 series, etc 
and 3" (not 3.5") -- Hitachi, etc) was to have a removeable 150Ohm 
termination resistor package. This was fitted on the last drive on the 
interface cale (that is the drive furthest from the controller) only, and 
acted both as a pull-up for the open-collector drivers used and as a 
termination for the cable. So of course the driver chips have to be able 
to sink 5V/150Ohms = 33mA

3.5" drives and later 5.25" ones have non-removable pull-up resistors (I 
won't call them terminators, they don't even attempt to match the 
characteristic impedance of the cable). These are normally about 1k (I've 
seen various values), and are fitted on all drives. Of course if you have 
2 drives on the cable, uou get the 2 pull-up resistors in parallel, and 
so on.

Electrically, this is the wrong thing to do (the cable should be 
terminated), but it's probably OK you have a shert interface cable (the 
drives are in a PC cabinet along with the controller. It suprisies me, 
actually, that even early 3.5" drives (like the full-height Sony units) 
didn't ahve proepr termination resistors. 

Another issue sprints to mind. The termination resisotr pack in the 
drive terminated/pulled up the signals from the controller to the drive 
only. The signals from drive to controller were terminated by 150 ohm 
rsistors on the cotnroller PCB. So the drive outputs have to be able to 
sink 33mA too. I know that the full-height Sony 3.5" drives used 7438s to 
drive these lines, and coule sink that current. What about more modern 
drives? Is there a problem with linking those to a 'classic' controller 


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