Rather than controlling the FDC directly, disk unit is an intelligent type that use it's own Z80 CPU and 8255 IC to communicate.
PC-8031 is a 5-inch 1D drive, and PC-8031-2W is a 5-inch 2D drive.
PC-80S31 is a compact type of PC-8031-2W and seems to have a slightly different timing. FDC uses μPD765A.