SCSI 68-Conductor Quiet

Cable Pair Location

The 68-conductor wide SCSI cable — designated the P cable in SCSI Parallel Interface (SPI) standards and the B cable in the earlier SCSI-2 specification (ANSI X3.131-1994) — carries a 16-bit data bus (DB0–DB15) together with two parity lines (DBP, DBP1) and the full set of SCSI control signals. The high-density 68-pin (HD68) connector operates at 0.05 in (1.27 mm) pitch and is used on Wide, Ultra, Ultra2, Ultra160, and Ultra320 SCSI drives and host adapters. In single-ended (SE) mode the cable is limited to 6 m; high-voltage differential (HVD, RS-485) extends that to 25 m; low-voltage differential (LVD) introduced with Ultra2 permits up to 12 m. Pins 1–34 carry ground or power in SE mode, with signals occupying pins 35–68; the differential column repurposes the lower pins as positive-sense signal pairs.

Notes

• The HD68 connector uses 0.05 in (1.27 mm) pitch contacts; the matching cable is a 68-conductor flat ribbon or 34 twisted-signal-pair construction. In the twisted-pair variant each signal conductor is paired with a dedicated ground return, which is the origin of the term “quiet cable”.
• Pins 17–18 and 51–52 carry TERMPWR (+5 V). SE termination uses 220 Ω to TERMPWR and 330 Ω to GND at each cable end; TERMPWR is supplied by the initiator through a blocking diode.
• Pin 16 carries DIFFSENSE (DIFFSNS) in differential mode. A differential SCSI device monitors this pin: if it is pulled to ground by an SE device on the bus, the HVD drivers are disabled to prevent damage. LVD devices use the same pin for the same protection function.
• In HVD mode the cable impedance is 122 Ω characteristic; in SE mode stub length must not exceed 0.1 m, and in HVD mode stubs must not exceed 0.2 m.
• Pin 19 and pin 53 are RESERVED and must not be grounded or driven by end devices.
• Wide SCSI adds the upper byte DB8–DB15 and a second parity bit DBP1 (labelled -PARITY1 in the SE column). Narrow (8-bit) SCSI devices ignore these signals.

Signals

• GROUND — signal return / shield; pins 1–16, 20–21, 30, 49–50 (SE mode lower pins) provide dedicated ground returns
• TERMPWR — terminator power rail (+5 V); supplied by the initiator through a blocking diode; used to bias SE terminators
• RESERVED — reserved contact; must not be connected
• DIFFSENSE — differential sense (pin 16); pulled low by SE devices to disable HVD drivers and prevent bus damage
• -DB0 – -DB7 — active-low data bus, lower byte (bits 0–7); asserted low = logic 1
• -DB8 – -DB15 — active-low data bus, upper byte (bits 8–15); wide SCSI extension
• -PARITY — active-low bus parity for lower data byte (DBP)
• -PARITY1 — active-low bus parity for upper data byte (DBP1); wide SCSI only
• -ATN — Attention; initiator signals target to enter Message Out phase
• -BSY — Busy; asserted by any device currently using the bus
• -ACK — Acknowledge; initiator handshake response to REQ during information transfer
• -RST — Reset; asserts a SCSI bus reset condition; all devices must go to BUS FREE
• -MSG — Message; target asserts during Message phase
• -SEL — Select; used during Selection and Reselection phases to address a target or initiator
• -C/D — Control/Data; distinguishes control (message/command/status) from data phases
• -REQ — Request; target handshake signal during information transfer
• -I/O — Input/Output; target indicates transfer direction relative to the initiator

References

• Small Computer System Interface – 2 (SCSI-2), Working Draft Revision 10L (X3T9.2 Project 375D, 7-SEP-93)
• Seagate SCSI Interface Product Manual, Volume 2, Rev. A (Publication 75789509)
• Seagate Medalist Pro Family Product Manual, Rev. D (ST39140/ST36530/ST34520)

Category:Data Storage Cables