23 PIN D-SUB FEMALE at the Amiga.
The Amiga external disk drive port uses a 23-pin D-sub female socket (DB23S) mounted on the Amiga itself, carrying a subset of Shugart-derived MFM floppy signals together with +5 V and +12 V power on the same connector. Documented in Appendix E of the Amiga Hardware Reference Manual (Commodore-Amiga Inc., 1991), the interface supports up to three external drives in a daisy-chain of maximum one metre total cable length, with power supplied only to the first device in the chain. Open-collector outputs require external 1000-ohm pull-up resistors, and the connector accommodates both 3.5-inch and 5.25-inch double-density MFM drives. Amiga-specific signals not found in the Shugart standard include /CHNG (disk-change latch), /DRESB (system reset propagation), and /MTRXD (motor control clocked into a drive flip-flop).
| Pin | Name | Dir | Description |
|---|---|---|---|
| 1 | /RDY | IN/OUT | Disk Ready |
| 2 | /DKRD | IN | Disk Read Data |
| 3 | GND | - | Ground |
| 4 | GND | - | Ground |
| 5 | GND | - | Ground |
| 6 | GND | - | Ground |
| 7 | GND | - | Ground |
| 8 | /MTRXD | OC | Disk Motor Control |
| 9 | /SEL2 | OC | Select Drive 2 |
| 10 | /DRES | OC | Disk Reset |
| 11 | /CHNG | IN/OUT | Disk Removed From Drive-Latched Low |
| 12 | +5V | OUT | +5 Volts DC (250 mA max) |
| 13 | /SIDE | OUT | Select Disk Side (0=Upper, 1=Lower) |
| 14 | /WPRO | IN/OUT | Disk is Write Protected |
| 15 | /TKO | IN/OUT | Drive Head position over Track 0 |
| 16 | /DKWE | OC | Disk Write Enable |
| 17 | /DKWD | OC | Disk Write Data |
| 18 | /STEP | OC | Step the Head-Pulse, First low, then high |
| 19 | DIR | OC | Select Head Direction (0=Inner, 1=Outer) |
| 20 | /SEL3 | OC | Select Drive 3 |
| 21 | /SEL1 | OC | Select Drive 1 |
| 22 | /INDEX | OC | Disk Index Pulse |
| 23 | +12V | OUT | +12 Volts DC (160 mA max, 540 mA surge |
Notes
- Direction is Computer relative Diskdrive.
- All open-collector (OC) outputs require 1000-ohm pull-up resistors on the drive side.
- Maximum daisy-chain cable length is one metre total, including all inter-drive segments; a maximum of three external drives may be connected.
- The +5 V supply (pin 12) is rated 270 mA continuous, 410 mA surge; +12 V (pin 23) is rated 160 mA continuous, 540 mA surge. Both power rails are supplied only to the first external device in the daisy-chain — subsequent drives must have their own power supply.
- The /CHNG latch is set at power-up and whenever no disk is installed; drives must reset it by pulsing /SEL when a disk is inserted.
- The connector on the Amiga is female (DB23S socket); the mating plug on the drive cable is a DB23 male.
Signals
- /RDY — Disk ready: asserted when the drive motor is on, a disk is installed, and the drive is up to speed.
- /DKRD — MFM-encoded read data stream from the drive to the Amiga’s Paula chip.
- GND — Signal ground (pins 3–7).
- /MTRXD — Motor control data, clocked into the drive’s motor on/off flip-flop by the SEL pulse.
- /SEL1, /SEL2, /SEL3 — Drive select for drives 1, 2, and 3 respectively; also used to clock /MTRXD into the motor flip-flop of the selected drive.
- /DRES — System reset propagation; causes drives to reset their motor-on flip-flops.
- /CHNG — Disk-change latch; indicates that no disk is present or that the disk was removed since the last SEL pulse.
- +5V — +5 V DC power to the first external device.
- /SIDE — Side select: selects the upper (0) or lower (1) recording surface.
- /WPRO — Write-protect indicator; asserted by the drive when the disk’s write-protect tab is engaged.
- /TKO — Track-zero indicator; asserted when the read/write head is positioned over track 0.
- /DKWE — Write gate enable; must be asserted before write data is accepted by the drive.
- /DKWD — MFM-encoded write data stream from the Amiga’s Paula chip to the drive.
- /STEP — Head-step pulse; transitions from low to high to advance the head by one track.
- DIR — Step direction: selects inward (0, higher track numbers) or outward (1, lower track numbers) head movement.
- /INDEX — Once-per-revolution index pulse generated by the drive; used by Paula for track synchronisation.
- +12V — +12 V DC power to the first external device.
References
- Amiga Hardware Reference Manual, 3rd Edition — Commodore-Amiga Inc., 1991; Appendix E documents the external disk connector interface specification.