20 pin Apple HDI-20 connector at the Apple computer, looking into cable-side connector.
The HDI-20 (High Density Interface, 20-pin) is a compact connector introduced by Apple for the PowerBook Duo series and its associated Duo MiniDock peripheral, enabling connection of a 1.4 MB SuperDrive floppy via an external floppy adapter. It is the successor to the DB-19 external floppy connector found on earlier classic Macintosh models from the 128K through the IIvx and Performa 600, carrying the same logical signals — power rails, phase/command lines, write request, head select, drive enable, and serial read/write data — but in a more compact high-density form. The SWIM (Super Woz Integrated Machine) chip, present in later classic Macs and the Duo-era hardware, controls the floppy interface and supports both GCR and MFM encoding, the latter required for the 1.4 MB HD format.
| Pin | Name | Description |
|---|---|---|
| 1 | GND | Signal Ground |
| 2 | GND | Signal Ground |
| 3 | GND | Signal Ground |
| 4 | GND | Signal Ground |
| 5 | NC | No connection |
| 6 | +5V | +5 volts DC |
| 7 | +5V | +5 volts DC |
| 8 | +5V | +5 volts DC |
| 9 | +5V | +5 volts DC |
| 10 | NC | No connection |
| 11 | PH0 | Phase 0: state control line |
| 12 | PH1 | Phase 1: state control line |
| 13 | PH2 | Phase 2: state control line |
| 14 | PH3 | Phase 3: register write strobe |
| 15 | WREQ | Write request |
| 16 | HDSEL | Head select |
| 17 | ENBL2 | External drive select |
| 18 | RD | Read data |
| 19 | WR | Write data |
| 20 | NC | No connection |
Signals
- GND — Signal ground; pins 1–4 provide four ground returns for the drive.
- NC — Not connected; pins 5, 10, and 20 are reserved and carry no signal.
- +5V — +5 volts DC supply; pins 6–9 provide four supply lines to the drive.
- PH0–PH2 — Phase/command control lines (also known as CA0–CA2 in Macintosh IWM nomenclature); used in combination with PH3/LSTRB to address and control the Sony drive’s internal state machine.
- PH3 — Phase 3 / register write strobe (LSTRB); used alongside PH0–PH2 to select and latch drive command registers.
- WREQ — Write request (active low); when asserted, the drive is enabled to accept serial write data.
- HDSEL — Head select; selects the upper or lower recording head on double-sided 3.5-inch media.
- ENBL2 — External drive select (active low); enables the read data line — RD is tri-stated when ENBL2 is deasserted.
- RD — Read data; carries GCR- or MFM-encoded serial data from the drive to the SWIM controller.
- WR — Write data; carries serial data from the SWIM controller to the drive for recording.
Notes
- The HDI-20 connector superseded the DB-19 external floppy connector on the PowerBook Duo and Duo MiniDock; the DB-19 remained in use on desktop Macintosh models up to the IIvx/Performa 600.
- Power pins (+5V on pins 6–9) are for the disk drive only and should not be used as a general supply rail.
- The SWIM chip operates at 15.6672 MHz in Macintosh mode (versus 14.3 MHz in Apple II mode), which governs the GCR encoding clock for 400 KB and 800 KB media, as well as MFM encoding for 1.4 MB HD media.