25 PIN D-SUB FEMALE to the Amiga/Mac)
50 PIN IDC FEMALE to the peripheral
Apple used a non-standard DB-25 connector for the external SCSI port on Macintosh computers, deviating from the 50-pin flat-ribbon connector specified by the ANSI SCSI standard. The same DB-25 pinout was adopted by Commodore for the Amiga. All 18 SCSI signals — eight data lines, one parity line, and nine control lines — appear on the DB-25, with six pins (7, 9, 14, 16, 18, and 24) serving as ground returns. The cable connecting a DB-25 host to a peripheral carrying a 50-pin IDC connector crosses these signals according to the mapping below; odd IDC pins (except pin 25) are ground.
| DSub | IDC | |
|---|---|---|
| Request | 1 | 48 |
| Message | 2 | 42 |
| Input/Output | 3 | 50 |
| Reset | 4 | 40 |
| Acknowledge | 5 | 38 |
| Busy | 6 | 36 |
| Data Bus 0 | 8 | 2 |
| Data Bus 3 | 10 | 8 |
| Data Bus 5 | 11 | 12 |
| Data Bus 6 | 12 | 14 |
| Data Bus 7 | 13 | 16 |
| Control/Data | 15 | 46 |
| Attention | 17 | 32 |
| Select | 19 | 44 |
| Data Parity | 20 | 18 |
| Data Bus 1 | 21 | 4 |
| Data Bus 2 | 22 | 6 |
| Data Bus 4 | 23 | 10 |
| Termination Power | 25 | 26 |
Notes
- DB-25 pins 7, 9, 14, 16, 18, and 24 are ground; they connect to all odd IDC pins except pin 25.
- IDC pin 25 must not be grounded at the host end; grounding it would short TERMPWR (IDC pin 26) to GND.
- All signals are active-low (asserted at low voltage, negated at high voltage), following the standard SCSI single-ended convention.
- On the Macintosh Plus, DB-25 pin 25 (TERMPWR) is not connected; termination power is handled internally. On all other Macintosh models, pin 25 supplies +5 V for terminator resistors only, not for powering SCSI peripherals.
- The Macintosh SCSI controller (NCR 5380 or 53C80) operates in single-ended (SE) mode; the SCSI bus uses standard passive termination (220 Ω to TERMPWR, 330 Ω to GND) at each end of the cable.
- Up to seven SCSI devices may be daisy-chained; the host logic board holds ID 7 (highest priority).
Signals
- Request — /REQ; asserted by the target to request a data transfer handshake from the initiator.
- Message — /MSG; asserted by the target during the Message phase to transfer command or status bytes.
- Input/Output — /I/O; set by the target to indicate the direction of data flow (low = input to initiator).
- Reset — /RST; resets all SCSI devices on the bus when asserted.
- Acknowledge — /ACK; asserted by the initiator to acknowledge a /REQ handshake from the target.
- Busy — /BSY; asserted by any device to indicate the bus is in use.
- Data Bus 0–7 — /DB(0)–/DB(7); eight active-low data lines forming the 8-bit SCSI data bus.
- Control/Data — /C/D; indicates whether the current bus phase is a Control phase (high) or a Data phase (low).
- Attention — /ATN; asserted by the initiator to request the Message phase.
- Select — /SEL; used during arbitration and selection to identify the target device.
- Data Parity — /DBP; odd parity bit covering the eight data lines.
- Termination Power — TERMPWR; +5 V supplied by the initiator through a blocking diode to power terminator resistor networks.