Amiga External Diskdrive

23 PIN D-SUB FEMALE 23 pin D-sub female at the computer.

1 /RDY I/O Disk Ready If motor on, indicates disk installed and up to speed. If motor not on, identification mode.
2 /DKRD I Disk Read Data MFM input data to Amiga.
3 GND - Ground  
4 GND - Ground  
5 GND - Ground  
6 GND - Ground  
7 GND - Ground  
8 /MTRXD OC Disk Motor Control Motor on data, clocked into drive's motor-on flip-flop by the active transition of SELxB b)
9 /SEL2B OC Select Drive 2 (A2000 /SEL3B) a)  b)
10 /DRESB OC Disk Reset Amiga system reset. Drives should reset their motor-on flip-flops and set their write-protect flip-flops.
11 /CHNG I/O Disk Removed From Drive-Latched Low Nominally used as an open collector input. Drive's change flop is set at power up or when no disk is not installed. Flop is reset when drive is selected and the head stepped, but only if a disk is installed.
12 +5V - +5 Volts DC 270 mA maximum; 410 mA surge.
When below 3.75V, drives are required to reset their motor-on flops, and set their write-protect flops.
13 /SIDE O Select Disk Side Side 1 if active, side 0 if inactive.
14 /WPRO I/O Disk is Write Protected Asserted by selected, write-protected disk.
15 /TK0 I/O Drive Head position over Track 0 Asserted by selected drive when read/write head is positioned over track 0.
16 /DKWE OC Disk Write Enable Write gate (enable) to drive.
17 /DKWD OC Disk Write Data MFM output data from Amiga.
18 /STEP OC Step the Head-Pulse, First low, then high Selected drive steps one cylinder in the direction indicated by DIR
19 DIR OC Select Head Direction (0=Inner, 1=Outer) Direction to step the head, Inactive to step towards center of the disk (higher-numbered tracks).
20 /SEL3 OC Select Drive 3 (A2000 not used) a)  b)
21 /SEL1 OC Select Drive 1 (A2000 /SEL2B) a)  b)
22 /INDEX I/O Disk Index Pulse Index is a pulse generated once per disk revolution, between the end and beginning of cylinders. The 8520 can be programmed to conditionally generate a level 6 interrupt to the 68000 whenever the /INDEX input goes active.
23 +12V - +12 Volts DC 160 mA max; 540 mA surge.

I = Input, O=Output, OC=Open Collector

a) /SEL1 is not drive 1, but rather the first external drive. Not all select lines may be implemented.
b) The drive select lines are shifted as they pass through a string of daisy chaned devices.

Source: Amiga Hardware Reference Manual