Parts taken from the Amiga 4000 Hardware Guide by Warren Block
Pin 1: *Reset Pin 21: Unused
Pin 2: Ground Pin 22: Ground
Pin 3: Drive Data 7 Pin 23: *I/O Write
Pin 4: Drive Data 8 Pin 24: Ground
Pin 5: Drive Data 6 Pin 25: *I/O Read
Pin 6: Drive Data 9 Pin 26: Ground
Pin 7: Drive Data 5 Pin 27: I/O Channel Ready
Pin 8: Drive Data 10 Pin 28: Unused
Pin 9: Drive Data 4 Pin 29: Unused
Pin 10: Drive Data 11 Pin 30: Ground
Pin 11: Drive Data 3 Pin 31: Interrupt
equest
Pin 12: Drive Data 12 Pin 32: Unused
Pin 13: Drive Data 2 Pin 33: Disk Address 1
Pin 14: Drive Data 13 Pin 34: Unused
Pin 15: Drive Data 1 Pin 35: Disk Address 0
Pin 16: Drive Data 14 Pin 36: Disk Address 2
Pin 17: Drive Data 0 Pin 37: *IDE_CS1
Pin 18: Drive Data 15 Pin 38: *IDE_CS2
Pin 19: Ground Pin 39: *Active (LED driver output.)
Pin 20: Unused Pin 40: Ground
For completeness, the extra four pins used on the mini 44-pin 2.5-inchIDE connector (as found on the A600 and A1200):
Pin 41: +5 volts
Pin 42: +5 volts
Pin 43: Ground
Pin 44: Unused