Amiga External Diskdrive
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
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