A2Aviator wrote:
http://twitpic.com/ijlx2 4th row, last picture- http://apple2.org/images/Interface_Cards/Floppy_Controllers/
That looks a lot like a Lobo-branded card I have here. I know there are a couple of driver diskettes for it and may even be able to find them :-).
The SVA Megaflex was a totally different device. It was physically smaller and supported DD 8" diskettes. The data rate on these was just fast enough (500Kbps) to make conventional polled I/O impossible, so vendors resorted to a number of techniques. The Megaflex established two almost-identical polling loops one page apart in memory. After issuing a read command to the FDC, the unit entered an endless loop. When the data-ready line went true, it pulled the A7 line in between instruction fetches. The CPU started executing a second loop from which it immediately branched out and starting reading bytes. Repeat, rinse...
Other DSDD controllers used buss-master DMA (Vista A800) or played tricks with halting the processor (Taurus).
Steve