SCSI External Centronics 50 (Differential)

50-pin Centronics external SCSI connector wired for HVD differential signalling, carrying complementary +/− signal pairs over 8-bit narrow SCSI at up to 25 m cable length.

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The external Centronics 50-pin connector (also known as CN50) is the standard external cable connector for narrow 8-bit SCSI-1 and SCSI-2 interfaces, defined in ANSI X3.131-1994 (SCSI-2) as a shielded low-density alternative at 2.16 mm contact pitch. This variant carries High Voltage Differential (HVD) signalling, in which each SCSI signal is transmitted as a complementary pair of +/− lines rather than a single line referenced to ground, as used in single-ended implementations. HVD operates at 5-volt logic and supports cable lengths of up to 25 m, compared to just 6 m for single-ended, making it suited to storage installations requiring long cabling. HVD was declared obsolete in SPI-3 (INCITS 336-2000) and is incompatible with both SE and LVD devices; the maximum achievable speed on an HVD bus was Ultra (20 MT/sec).

50 PIN CENTRONICS FEMALE 50 PIN CENTRONICS FEMALE at the controller & devices

50 PIN CENTRONICS MALE 50 PIN CENTRONICS 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 data bus bits 0–7 (8-bit narrow SCSI). The positive and negative lines of each pair are driven to inverse voltages by HVD transceivers.
  • +DBP / −DBP — Differential pair for the odd parity bit covering the 8-bit data byte.
  • DIFFSENS — Differential Sense pin (pin 11). Used by LVD/MSE multimode devices to detect the bus signalling type; on a pure HVD bus this pin is held at a voltage that identifies the bus as differential. HVD-only devices do not actively drive this pin.
  • TERMPWR — Termination Power (pins 13 and 38). Supplies voltage to terminator networks at each end of the bus; present on two pins for current redundancy.
  • +ATN / −ATN — Differential Attention signal; asserted by an initiator to request a Message Out phase.
  • +BSY / −BSY — Differential Bus Busy; wired-OR across all devices; asserted whenever the bus is in use.
  • +ACK / −ACK — Differential Acknowledge; asserted by an initiator to acknowledge each data or command byte transferred.
  • +RST / −RST — Differential Reset; resets all devices on the bus when asserted.
  • +MSG / −MSG — Differential Message; asserted by a target to indicate the Message phase.
  • +SEL / −SEL — Differential Select; used during arbitration and selection/reselection phases.
  • +C/D / −C/D — Differential Control or Data; distinguishes command/status/message bytes (asserted) from data bytes (negated).
  • +REQ / −REQ — Differential Request; asserted by a target to request each byte transfer in the synchronous or asynchronous data phase.
  • +I/O / −I/O — Differential In/Out; indicates transfer direction relative to the initiator (asserted = data flowing into initiator).
  • res (pins 12, 14, 37, 39) — Reserved; must not be connected.
  • GND (pins 1, 16, 25, 26, 36, 41, 50) — Signal ground and cable shield reference.

Notes

  • Direction is device-relative to the bus (other devices).
  • This connector carries HVD (High Voltage Differential) signalling. HVD uses 5-volt complementary +/− line pairs for each signal, enabling cable runs up to 25 m. It is electrically incompatible with Single-Ended (SE) and Low Voltage Differential (LVD) devices; connecting an HVD device to an SE or LVD bus will damage the equipment.
  • HVD was formally declared obsolete in SPI-3 (INCITS 336-2000). Maximum bus speed on HVD is Ultra (20 MT/sec / 20 MB/sec for 8-bit narrow).
  • HVD buses use passive HVD terminators exclusively; active SE terminators must not be fitted.
  • Exactly two terminators are required per SCSI bus segment, fitted at the two extreme ends only. Stub length must be kept below 0.2 m (8 in) per tap.
  • The Centronics 50-pin external shell is mechanically keyed and latches with wire bails. It is distinct from the internal 50-pin IDC header (2.54 mm pitch) and the high-density 50-pin (DH50, 1.27 mm pitch) used in SCSI-2.
  • TERMPWR is present on two pins (13 and 38) to share current load. It must be supplied by at least one device on the bus (typically the host adapter).

References

Category:Buses Connectors