SCSI Internal (Differential)

50-pin IDC ribbon connector for internal HVD differential SCSI, carrying 8-bit narrow SCSI signals on complementary +/- line pairs over up to 25 m.

Updated:

SCSI = Small Computer System Interface

SCSI Internal (Differential) – 50-pin IDC header 50 PIN IDC MALE at the controller and devices

SCSI Internal (Differential) – 50-pin IDC header 50 PIN IDC FEMALE at the cable

Internal differential SCSI uses the same 50-pin IDC (Insulation Displacement Connector) ribbon connector as single-ended internal SCSI, but employs High Voltage Differential (HVD) electrical signalling as defined in the SCSI-2 standard (ANSI X3.131-1994) and carried forward into the SPI-2 specification (INCITS 302-1998). HVD drives each signal as a complementary +/- pair on two lines, enabling cable runs of up to 25 m — substantially longer than the 6 m maximum for single-ended configurations. The bus carries an 8-bit narrow data path (DB0–DB7 plus parity) suited to SCSI-1 and SCSI-2 devices operating at speeds up to Ultra (20 MT/sec); HVD was formally declared obsolete in SPI-3 (INCITS 336-2000), making true internal HVD installations rare in practice.

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

Signals

  • +DBn / -DBn — Complementary HVD differential pair for data bit n (DB0–DB7); 8-bit narrow data bus.
  • +DBP / -DBP — Complementary differential pair for data bus parity (odd parity).
  • DIFFSENS — Differential Sense pin; used by devices to detect the bus signalling type (HVD or LVD) and prevent accidental mixing of incompatible electrical variants. HVD and LVD are electrically incompatible and must never be connected to the same bus.
  • TERMPWR — Termination Power; supplies voltage to the bus terminators; two pins (25 and 26) are provided.
  • +ATN / -ATN — Attention; asserted by the initiator to signal a message to the target.
  • +BSY / -BSY — Bus Busy; indicates the SCSI bus is in use.
  • +ACK / -ACK — Acknowledge; handshake signal from initiator confirming data transfer.
  • +RST / -RST — Reset; resets all devices on the bus.
  • +MSG / -MSG — Message phase indicator, driven by the target.
  • +SEL / -SEL — Select; used during arbitration and selection phases.
  • +C/D — Control/Data phase indicator, driven by the target; high = control, low = data.
  • -C/D — Complement of C/D for HVD differential signalling.
  • +REQ / -REQ — Request; handshake signal from target initiating a data transfer.
  • +I/O / -I/O — In/Out phase indicator; determines the direction of data transfer relative to the initiator.
  • res — Reserved; must not be connected.

Notes

  • Direction is relative to the device on the bus (other devices).
  • The 50-pin IDC ribbon connector uses two rows of 25 pins on 0.1 inch (2.54 mm) centres; this is the Alternative 2 low-density non-shielded connector defined in SCSI-2 (ANSI X3.131-1994).
  • HVD differential signalling uses complementary +/- line pairs per signal, enabling cable lengths of up to 25 m — compared to 6 m for single-ended and 12 m for LVD.
  • HVD operates at 5 V logic; it is electrically incompatible with both LVD (3.3 V) and single-ended configurations. Connecting HVD devices to a single-ended or LVD bus will cause damage.
  • HVD was the maximum speed of Ultra SCSI (20 MT/sec) and was declared obsolete in SPI-3 (INCITS 336-2000); no HVD devices were produced for Ultra2 or later standards.
  • Exactly two terminators are required per bus segment, fitted only at the two extreme ends; HVD buses require passive HVD terminators (differential terminator impedance 122 ohms).
  • All odd-numbered pins in the IDC header are grounds except pin 25 (TERMPWR) and pin 21 (DIFFSENS). This pattern provides shielding between signal pairs along the ribbon cable.

References

Category:Buses Connectors