40 pin IDC male connector at the controller and peripherals (AMP 6-176135).
40 pin IDC female connector at the cable.
The 2.5-inch internal SCSI 40-pin connector is a compact IDC (Insulation Displacement Contact) header used on notebook and laptop hard drives in place of the separate 50-pin data and Molex power connectors found on 3.5-inch drives. It carries narrow 8-bit SCSI signals alongside integrated +5 V power lines, making it a self-contained interface suited to space-constrained portable systems from the early-to-mid 1990s. Unlike 3.5-inch internal SCSI drives, no separate power connector is required; pins 1–2 and 39–40 supply +5 V, while pin 17 is a physical key position (absent from the housing) to prevent misorientation. This connector variant is specified in drive manufacturer SFF documents rather than the main ANSI SCSI-2 standard (ANSI X3.131-1994), which defines only the standard 50-pin narrow and 68-pin wide internal connectors.
| Pin | Name | Dir | Description |
|---|---|---|---|
| 1 | +5V | - | +5 VDC |
| 1 | +5V | - | +5 VDC |
| 1 | RET | - | Return (Ground?) |
| 1 | RET | - | Return (Ground?) |
| 1 | GND | IN/OUT | Ground |
| 2 | DB0 | IN/OUT | Data Bus 0 |
| 1 | GND | IN/OUT | Ground |
| 4 | DB1 | IN/OUT | Data Bus 1 |
| 1 | GND | IN/OUT | Ground |
| 6 | DB2 | IN/OUT | Data Bus 2 |
| 1 | GND | IN/OUT | Ground |
| 8 | DB3 | IN/OUT | Data Bus 3 |
| 1 | GND | IN/OUT | Ground |
| 10 | DB4 | IN/OUT | Data Bus 4 |
| 1 | GND | IN/OUT | Ground |
| 12 | DB5 | IN/OUT | Data Bus 5 |
| 1 | GND | IN/OUT | Ground |
| 14 | DB6 | IN/OUT | Data Bus 6 |
| 1 | GND | IN/OUT | Ground |
| 16 | DB7 | IN/OUT | Data Bus 7 |
| 1 | GND | IN/OUT | Ground |
| 18 | PARITY | IN/OUT | Data Parity (odd Parity) |
| 24 | GND | - | Ground |
| 26 | TMPWR | IN/OUT | Termination Power |
| 32 | /ATN | IN | Attention |
| 36 | /BSY | IN/OUT | Busy |
| 24 | GND | - | Ground |
| 38 | /ACK | IN | Acknowledge |
| 40 | /RST | IN/OUT | Reset |
| 42 | /MSG | OUT | Message |
| 24 | GND | - | Ground |
| 44 | /SEL | IN/OUT | Select |
| 50 | /I/O | OUT | Input/Output |
| 46 | /C/D | OUT | Control/Data |
| 24 | GND | - | Ground |
| 48 | /REQ | OUT | Request |
| 1 | RET | - | Return (Ground?) |
| 1 | RET | - | Return (Ground?) |
| 1 | +5V | - | +5 VDC |
| 1 | +5V | - | +5 VDC |
Signals
- +5V — +5 V DC supply integrated into the connector (pins 1–2 at one end and 39–40 at the other end of the 40-pin header); no separate power connector is used with 2.5-inch drives.
- RET — Return line for the power supply; functions as the power ground (distinct from the SCSI signal ground lines).
- GND — Signal ground; interspersed between data and control signal pairs to reduce crosstalk on the ribbon cable.
- DB0–DB7 — Eight data bus lines forming the narrow (8-bit) SCSI data path, carried on even-numbered pins.
- PARITY — Odd parity bit covering the 8-bit data bus (DB0–DB7); allows the target or initiator to detect single-bit transfer errors.
- TMPWR — Termination power; supplies voltage to the SCSI bus termination resistors at each end of the cable segment.
- /ATN — Attention (active-low); asserted by the initiator to request a Message Out phase from the target.
- /BSY — Busy (active-low); asserted by any device holding the SCSI bus.
- /ACK — Acknowledge (active-low); asserted by the initiator to confirm receipt of each data or message byte during a handshake cycle.
- /RST — Reset (active-low); when asserted, all devices on the bus immediately return to their initial state.
- /MSG — Message (active-low); driven by the target to indicate the Message phase is in progress.
- /SEL — Select (active-low); used during the Selection and Reselection phases to address a specific device on the bus.
- /I/O — Input/Output (active-low); driven by the target to indicate the direction of data transfer (low = data flowing to initiator).
- /C/D — Control/Data (active-low); driven by the target to distinguish control information (high) from data transfers (low).
- /REQ — Request (active-low); driven by the target to initiate each byte of a handshake transfer with the initiator.
Notes
- Key position: Pin 17 is absent from the male connector housing, forming a mechanical key that prevents incorrect insertion.
- Integrated power: Unlike 3.5-inch SCSI drives, which use a separate 4-pin Molex power connector, the 2.5-inch 40-pin header carries +5 V supply and return on pins 1–2 and 39–40. There is no +12 V rail; 2.5-inch drives operate on +5 V only.
- Signalling: This connector carries Single-Ended (SE) SCSI signals at 5 V logic levels. The 40-pin format supports only narrow (8-bit) SCSI; Wide (16-bit) SCSI requires a 68-pin connector.
- Termination: As with all SCSI bus segments, exactly two terminators are required — one at each extreme end of the cable. Passive termination (220/330 Ω resistor networks) is adequate for Slow SCSI (up to 5 MB/sec); active termination is required for Fast SCSI and above.
- Cable stub length: Single-ended SCSI stub length must be kept under 0.1 m (4 in) to avoid signal reflections at faster transfer rates.
- Standards context: This connector form factor is defined by drive manufacturer SFF (Small Form Factor) specifications rather than by ANSI X3.131-1994 (SCSI-2), which covers only the 50-pin IDC narrow and 68-pin wide internal connectors. The 40-pin variant is specific to 2.5-inch (notebook/laptop) storage devices.