SCSI = Small Computer System Interface
25 PIN D-SUB FEMALE at the controller
25 PIN D-SUB MALE at the cable.
The DB-25 external SCSI connector is a non-standard but widely adopted mapping of the 50-pin narrow SCSI signal set onto a 25-pin D-Sub shell, originating with Apple’s original Macintosh SCSI implementation. It was used on Apple Macintosh computers (pre-PowerBook), Commodore Amiga peripherals (including the A2091, GVP, and Phase V host adaptors), and certain Sun workstations. Because a DB-25 carries only 8-bit narrow SCSI (no Wide bus support), it is limited to Single-Ended signalling at Slow SCSI speeds — a maximum of 5 MB/sec — with cable lengths not exceeding approximately 1.2–1.5 m. There is no ANSI or INCITS standard governing this connector; the pin mapping derives entirely from Apple’s original implementation and was subsequently adopted by compatible third-party vendors.
| Pin | Name | Dir | Description |
|---|---|---|---|
| 1 | /REQ | OUT | Request |
| 2 | /MSG | OUT | Message |
| 3 | I/O | OUT | Input/Output |
| 4 | /RST | IN/OUT | Reset |
| 5 | /ACK | IN | Acknowledge |
| 6 | BSY | IN/OUT | Busy |
| 7 | GND | - | Ground |
| 8 | DB0 | IN/OUT | Data Bus 0 |
| 9 | GND | - | Ground |
| 10 | DB3 | IN/OUT | Data Bus 3 |
| 11 | DB5 | IN/OUT | Data Bus 5 |
| 12 | DB6 | IN/OUT | Data Bus 6 |
| 13 | DB7 | IN/OUT | Data Bus 7 |
| 14 | GND | - | Ground |
| 15 | C/D | OUT | Control/Data |
| 16 | GND | - | Ground |
| 17 | /ATN | IN | Attention |
| 18 | GND | - | Ground |
| 19 | /SEL | IN/OUT | Select |
| 20 | PARITY | IN/OUT | Data Parity |
| 21 | DB1 | IN/OUT | Data Bus 1 |
| 22 | DB2 | IN/OUT | Data Bus 2 |
| 23 | DB4 | IN/OUT | Data Bus 4 |
| 24 | GND | - | Ground |
| 25 | TMPWR | IN/OUT | Termination Power |
Signals
- /REQ — Request; asserted by the target device to initiate a data transfer phase.
- /MSG — Message; used with C/D and I/O to identify the Message phase on the bus.
- I/O — Input/Output; defines the direction of data transfer relative to the initiator.
- /RST — Reset; drives all devices on the bus to a known state when asserted.
- /ACK — Acknowledge; asserted by the initiator to handshake each byte transferred.
- BSY — Busy; indicates the bus is in use by one or more devices.
- GND — Signal ground; multiple ground pins reduce noise at Slow SCSI frequencies.
- DB0–DB7 — Eight data bus lines carrying the 8-bit narrow SCSI data byte (pins 8, 10–13, 21–23).
- C/D — Control/Data; distinguishes command/status/message bytes (high) from data bytes (low).
- /ATN — Attention; asserted by the initiator to request a Message Out phase.
- /SEL — Select; used by initiators and targets to identify themselves during selection and reselection.
- PARITY — Odd parity over DB0–DB7; optional in SCSI-1, mandatory from SCSI-2 onwards.
- TMPWR — Termination Power; supplies voltage to bus terminators at each end of the cable segment.
Notes
- Direction is device relative to bus (other devices).
- This connector carries 8-bit (narrow) SCSI signals only; no Wide (16-bit) bus support is possible via a DB-25.
- Signalling is Single-Ended (SE) at 5 V logic; High Voltage Differential (HVD) and Low Voltage Differential (LVD) are incompatible with this connector type.
- Maximum cable length is approximately 1.2–1.5 m (Slow SCSI, 5 MB/sec). Exceeding this length risks signal integrity issues.
- Two terminators are required per bus segment — one at each extreme end. Passive termination (220/330 Ω) is acceptable at Slow SCSI speeds.
- The pin mapping is not an ANSI/INCITS standard; it originates with Apple and was adopted by Amiga and Sun third-party vendors. It is distinct from the Future Domain DB-25 mapping (used only on TMC-series adaptors) and must not be confused with it.
- An adapter cable is required to connect this DB-25 port to a standard 50-pin Centronics (CN50) or 50-pin high-density (DH50) SCSI device.