The Future Domain DB-25 is a proprietary external SCSI connector that maps the standard 50-pin narrow SCSI signals onto a 25-pin D-Sub shell. Unlike the physically identical Apple/Mac/Amiga DB-25 SCSI connector, this variant uses a different pin assignment and was fitted exclusively to Future Domain TMC-830/845 and TMC-850/860/885 host adapters; it is not interchangeable with any other DB-25 SCSI implementation. Being a narrow (8-bit) single-ended interface, it is limited to Slow SCSI speeds and short cable runs. No ANSI or INCITS standard governs this mapping; it is a purely proprietary arrangement defined by Future Domain.
25 PIN D-SUB FEMALE at the controller
25 PIN D-SUB MALE at the cable
| Pin | Name | Dir | Description |
|---|---|---|---|
| 1 | GND | - | Ground |
| 2 | DB1 | IN/OUT | Data Bus 1 |
| 3 | DB3 | IN/OUT | Data Bus 3 |
| 4 | DB5 | IN/OUT | Data Bus 5 |
| 5 | DB7 | IN/OUT | Data Bus 7 |
| 6 | GND | - | Ground |
| 7 | /SEL | IN/OUT | Select |
| 8 | GND | - | Ground |
| 9 | TMPWR | IN/OUT | Termination Power |
| 10 | /RST | IN/OUT | Reset |
| 11 | C/D | OUT | Control/Data |
| 12 | I/O | OUT | Input/Output |
| 13 | GND | - | Ground |
| 14 | DB0 | IN/OUT | Data Bus 0 |
| 15 | DB2 | IN/OUT | Data Bus 2 |
| 16 | DB4 | IN/OUT | Data Bus 4 |
| 17 | DB6 | IN/OUT | Data Bus 6 |
| 18 | PARITY | IN/OUT | Data Parity |
| 19 | GND | - | Ground |
| 20 | /ATN | IN | Attention |
| 21 | /MSG | OUT | Message |
| 22 | /ACK | IN | Acknowledge |
| 23 | BSY | IN/OUT | Busy |
| 24 | /REQ | OUT | Request |
| 25 | GND | - | Ground |
Signals
- GND — Signal ground; pins 1, 6, 8, 13, 19, and 25 are all ground returns.
- DB0–DB7 — Eight single-ended data bus lines (8-bit narrow SCSI); all signals are active-low per the SCSI-1/SCSI-2 convention.
- PARITY — Odd parity bit for the 8-bit data bus; active-low.
- /SEL — Select: used by an initiator to select a target device on the bus; active-low.
- /RST — Reset: asserted to reset all devices on the SCSI bus; active-low.
- C/D — Control/Data: driven by the target to indicate whether the current bus phase is carrying control information or data.
- I/O — Input/Output: driven by the target to indicate the direction of data transfer relative to the initiator.
- TMPWR — Termination Power: supplies voltage to bus terminators; equivalent to TERMPWR on standard connectors.
- /ATN — Attention: asserted by the initiator to request a message exchange; active-low.
- /MSG — Message: driven by the target to indicate a message phase; active-low.
- /ACK — Acknowledge: asserted by the initiator to acknowledge each data or message byte transferred; active-low.
- BSY — Busy: asserted by any device currently controlling the bus; active-low.
- /REQ — Request: asserted by the target to request a data or message transfer handshake; active-low.
Notes
- Direction is device-relative to the bus (other devices).
- This DB-25 pin mapping is unique to Future Domain TMC-series host adapters (TMC-830, TMC-845, TMC-850, TMC-860, TMC-885) and is not compatible with the Apple/Mac/Amiga DB-25 SCSI mapping.
- The connector is single-ended (SE) only. Maximum cable length for Slow SCSI (up to 5 MB/sec) is 6 m; in practice, the DB-25 shell’s limited shielding makes shorter runs advisable.
- Two terminators are required per SCSI bus segment, placed at the two extreme ends. Passive termination (220/330 ohm) is acceptable at Slow SCSI speeds.
- TMPWR (pin 9) supplies termination power; ensure the host adapter or an external terminator provides this voltage.
- No DB pin carries a differential complement; this connector supports single-ended signalling exclusively and cannot be used on an HVD or LVD bus.