CN13: FDD CONN

Clearly this is a Floppy disk interface, but not the usual one with 34 pins, but one with 26 pins as you find in laptops. The table below show the comparison between a normal floppy interface and CN13 together with the pins connected to the I/O chip PC87309IBW/VLJ and the signal names.
 

FDD    CN13     U21     SIGNAL

  2       13   97/48/44  DENSITY SELECT (DENSEL)
  4       -       -     not used
  6       -       -     not used
  8       2      56     INDEX
 10      10      45     MOTOR ENABLE 0
 12       -      48     DRIVE SELECT 1
 14       4      47     DRIVE SELECT 0
 16       -      46     MOTOR ENABLE 1
 18      12      50     DIRECTION (DIR)
 20      14      51     STEP
 22      16      49     WRITE DATA (WD)
 24      18      53     WRITE GATE (WE)
 26      20      55     TRACK0
 28      22      57     WRITE PROTECT (WP)
 30      24      54     READ DATA (RD)
 32      26      52     HEAD SELECT (HDSEL)
 34       6      58     DISK CHANGE
  -       -    95/45/43 DATA RATE0 
uneven pins on fdd connector are ground signal.
pins 1,3,5 of CN13 are connected to each other on the main board and
should supply the power for the drive.
pins 7,8,9,11 are unconnected.
pins 15,17,19,21,23,25 are VSS/ground signal.