FC5025 USB 5.25 floppy controller - first report

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 board? -tony