Did anybody have SED9421 datasheet? This is data separator from floppy drive controller Yamaha FD-051.
I have only SED9420 datasheet, but this ICs is difference :-(
There are only one "russian" FDC ;-)
Many Yamaha YIS503IIR and YIS503IIIR was installed in soviet schools.
FD-051 was a standard FDC for teacher's computer.
Copy of FD-051 is working now, but I am not sure about capacitors value for data separator.
SED9420 is a FDC data separator. You can find datasheet in the web.
SED9421 should be a later release with faster speed or support for more disk formats. I am not sure.