SCSI-II External Hi D-Sub (Differential)

50-pin high-density D-Sub external connector for SCSI-2 High Voltage Differential (HVD) signalling, supporting cable runs up to 25 m.

Updated:

The SCSI-II External Hi D-Sub (Differential) is a 50-pin high-density D-Sub connector used for external High Voltage Differential (HVD) SCSI-2 cabling, as defined in ANSI X3.131-1994 and elaborated in INCITS 302-1998 (SPI-2). Operating at 5-volt logic with complementary +/− signal pairs for each bus line, HVD allows cable lengths of up to 25 m — far exceeding the 6 m limit of single-ended SCSI. The connector mates a 50-pin high-density D-Sub female at each device or controller with a corresponding male on the cable, carrying the full 8-bit narrow SCSI signal set including data, parity, and all nine control lines in differential form. HVD was formally declared obsolete in SPI-3 (INCITS 336-2000) and must not be mixed with LVD or SE devices on the same bus.

50 PIN HI-DENSITY D-SUB FEMALE 50 PIN HI-DENSITY D-SUB FEMALE at the controller and devices

50 PIN HI-DENSITY D-SUB MALE 50 PIN HI-DENSITY D-SUB MALE at the cable

PinNameDirDescription
01GND-Ground
02+DB0IN/OUT+Data Bus 0
03+DB1IN/OUT+Data Bus 1
04+DB2IN/OUT+Data Bus 2
05+DB3IN/OUT+Data Bus 3
06+DB4IN/OUT+Data Bus 4
07+DB5IN/OUT+Data Bus 5
08+DB6IN/OUT+Data Bus 6
09+DB7IN/OUT+Data Bus 7
10+DBPIN/OUT+Data Bus Parity (odd Parity)
11DIFFSENS????
12res-Reserved
13TERMPWRIN/OUTTermination Power
14res-Reserved
15+ATNIN+Attention
16GND-Ground
17+BSYIN/OUT+Bus is busy
18+ACKIN+Acknowledge
19+RSTIN/OUT+Reset
20+MSGOUT+Message
21+SELIN/OUT+Select
22+C/DOUT+Control or Data
23+REQOUT+Request
24+I/OOUT+In/Out
25GND-Ground
26GND-Ground
27-DB0IN/OUT-Data Bus 0
28-DB1IN/OUT-Data Bus 1
29-DB2IN/OUT-Data Bus 2
30-DB3IN/OUT-Data Bus 3
31-DB4IN/OUT-Data Bus 4
32-DB5IN/OUT-Data Bus 5
33-DB6IN/OUT-Data Bus 6
34-DB7IN/OUT-Data Bus Parity7
35-DBPIN/OUT-Data Bus Parity (odd Parity)
36GND-Ground
37res-Reserved
38TERMPWRIN/OUTTermination Power
39res-Reserved
40-ATNIN-Attention
41GND-Ground
42-BSYIN/OUT-Bus is busy
43-ACKIN-Acknowledge
44-RSTIN/OUT-Reset
45-MSGOUT-Message
46-SELIN/OUT-Select
47-C/DOUT-Control or Data
48-REQOUT-Request
49-I/OOUT-In/Out
50GND-Ground

Signals

  • +DB0 / −DB0 … +DB7 / −DB7 — complementary differential pairs carrying the 8 data bits of the narrow (8-bit) SCSI bus; the positive line is asserted low and the negative line inversely for HVD signalling.
  • +DBP / −DBP — odd-parity bit for the data byte, transmitted as a differential pair alongside the data lines.
  • DIFFSENS — DIFFSENSE pin used by devices to detect the bus signalling type (HVD or LVD/SE); its direction and voltage level are implementation-defined, hence the table reflects the signal’s indeterminate role in this context.
  • TERMPWR — termination power rail (typically +5 V) supplied to bus terminators; appears on both pins 13 and 38 to ensure adequate current delivery across the connector.
  • +ATN / −ATN — differential Attention signal; asserted by the initiator to request a Message Out phase from the target.
  • +BSY / −BSY — differential Bus Busy; wire-OR signal asserted by any device actively using the bus.
  • +ACK / −ACK — differential Acknowledge; handshake response from initiator during data transfer, paired with REQ.
  • +RST / −RST — differential Reset; drives all devices on the bus to a known initial state when asserted.
  • +MSG / −MSG — differential Message phase indicator; asserted by the target to signal a Message phase.
  • +SEL / −SEL — differential Select; used during arbitration and selection/reselection phases to identify the active device.
  • +C/D / −CD — differential Control/Data; distinguishes command, status, and message bytes (asserted) from data bytes (negated).
  • +REQ / −REQ — differential Request; asserted by the target to request a data transfer handshake with the initiator.
  • +I/O / −I/O — differential In/Out; indicates the direction of data transfer relative to the initiator.

Notes

  • Direction is device relative to bus (other devices).
  • This connector operates exclusively with High Voltage Differential (HVD) signalling at 5-volt logic levels. HVD uses complementary +/− line pairs for each signal to achieve high noise immunity and support cable runs of up to 25 m.
  • HVD termination uses passive HVD terminators with 122-ohm impedance; active termination is not applicable to HVD. Exactly two terminators must be fitted, one at each extreme end of the bus segment.
  • HVD and LVD devices are electrically incompatible and must never be connected to the same bus; connecting an LVD or SE device to an HVD bus can cause permanent hardware damage.
  • HVD was formally declared obsolete in SPI-3 (INCITS 336-2000); the maximum speed achieved under HVD was Ultra SCSI at 20 MT/sec.
  • The 50-pin high-density D-Sub shell is pitched at 1.27 mm (vs 2.54 mm for standard D-Sub), requiring HVD-rated high-density cables and connectors throughout.
  • Stub length for HVD must not exceed 0.2 m (8 in) between the bus backbone and any device drop cable.

References

Category:Buses Connectors