SCSI 50-Conductor Quiet

Cable Pair Location

The SCSI 50-conductor quiet cable is the twisted-pair form of the SCSI-2 A cable, defined in ANSI X3.131-1994 (commonly referred to as SCSI-2) by the ANSI Accredited Standards Committee X3/T10 Technical Committee. It carries an 8-bit data bus (DB0–DB7) plus parity and nine control signals over 25 twisted pairs, with each pair consisting of one signal conductor and one dedicated ground return — the ground return between each signal pair is the origin of the “quiet” designation. The cable connects to either a 50-pin high-density (HD50, 1.27 mm pitch) or a 50-pin low-density IDC/Centronics (2.54 mm pitch) connector, and supports both single-ended (SE) and high-voltage differential (HVD) electrical modes with a maximum cable length of 6 m (SE) or 25 m (HVD).

Notes

The table above shows cable-pair assignments for a 25-pair twisted cable. Each pair carries one signal and one dedicated ground return; in the single-ended column pins 1–25 are all ground, while pins 26–50 carry the active-low SCSI signals. In the differential (HVD) column each pair forms a balanced +/– signal, with the positive and negative conductors on the two pins of each twisted pair.

SE termination requires 220 Ω to TERMPWR and 330 Ω to GND at each end of the cable; TERMPWR (+5 V) is supplied by the initiator through a blocking diode. HVD termination uses a characteristic impedance of 122 Ω. Maximum SE cable length is 6 m; HVD extends to 25 m. Stubs must be kept to 0.1 m or less in SE mode.

Contact 25 (SE column: OPEN) must not be grounded at the host end, as doing so would short TERMPWR on pin 26 (38 in the connector) to ground.

DIFFSENSE (pin 11, cable pair 23, HVD column) is an active-high enable for differential drivers; it is pulled low if a single-ended device is detected on the bus, disabling HVD drivers to prevent damage.

Signals

• -DB0 – -DB7 — Active-low 8-bit data bus (SCSI data lines, narrow bus)
• -PARITY — Active-low odd parity bit for DB0–DB7
• +DB0 – +DB7 — Positive side of differential data pairs (HVD mode)
• +PARITY — Positive side of differential parity pair (HVD mode)
• -ATN — Attention: asserted by initiator to enter message-out phase
• -BSY — Busy: asserted by the device currently controlling the bus
• -ACK — Acknowledge: handshake response from initiator during data transfer
• -RST — Reset: asserted to force all devices to initial state
• -MSG — Message: indicates message phase when asserted together with -C/D
• -SEL — Select: used during selection and reselection phases
• -C/D — Control/Data: distinguishes control phase (asserted) from data phase (negated)
• -REQ — Request: handshake request from target during data transfer
• -I/O — Input/Output: indicates transfer direction relative to the initiator
• +ATN – +I/O — Positive sides of differential control signal pairs (HVD mode)
• -ATN – -I/O — Negative sides of differential control signal pairs (HVD mode)
• TERMPWR — Terminator power (+5 V); supplied by initiator through a blocking diode to power termination resistors
• DIFFSENSE — Differential sense: pulled high to enable HVD drivers; grounded by any SE device to disable them
• GROUND — Signal ground return
• RESERVED — Reserved for future use; must not be connected
• OPEN — Pin 25 in SE mode must remain unconnected at the host end

References

• Small Computer System Interface – 2 (SCSI-2), Working Draft X3T9.2 Revision 10L
• Seagate SCSI Interface Product Manual, Volume 2, Rev. A
• Seagate Medalist Pro Family Product Manual, Rev. D

Category:Data Storage Cables