SCSI-II External Hi D-Sub (Single-ended)

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01 Mar 1998

Updated: 29 Jun 2026

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

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

The SCSI-II External Hi D-Sub is the shielded, high-density 50-pin D-Sub (HD-50) connector used for external narrow SCSI-2 cabling. Defined in ANSI X3.131-1994 (SCSI-2) and carried forward in the SPI-2 physical-layer standard (INCITS 302-1998), it replaced the bulkier Centronics CN-50 for external use, offering contacts at 1.27 mm pitch in a compact D-shell housing. The connector carries an 8-bit data bus (DB0–DB7) with odd parity, plus the nine SCSI control signals, and uses single-ended (SE) 5 V signalling, limiting maximum cable length to 6 m (or 3 m for Ultra SCSI). Pins 1–25 are all ground, providing effective shielding for each signal return path; termination power is supplied on pin 38 (TERMPWR/TMPWR).

Pin	Name	Dir	Description
1-25	GND	-	Ground
26	DB0	IN/OUT	Data Bus 0
27	DB1	IN/OUT	Data Bus 1
28	DB2	IN/OUT	Data Bus 2
29	DB3	IN/OUT	Data Bus 3
30	DB4	IN/OUT	Data Bus 4
31	DB5	IN/OUT	Data Bus 5
32	DB6	IN/OUT	Data Bus 6
33	DB7	IN/OUT	Data Bus 7
34	PARITY	IN/OUT	Data Parity (odd Parity)
35	GND	-	Ground
36	GND	-	Ground
37	GND	-	Ground
38	TMPWR	IN/OUT	Termination Power
39	GND	-	Ground
40	GND	-	Ground
41	/ATN	IN	Attention
42	n/c	-	Not connected
43	/BSY	IN/OUT	Busy
44	/ACK	IN	Acknowledge
45	/RST	IN/OUT	Reset
46	/MSG	OUT	Message
47	/SEL	IN/OUT	Select
48	/C/D	OUT	Control/Data
49	/REQ	OUT	Request
50	/I/O	OUT	Input/Output

Notes

Direction is device relative to the bus (other devices).
This is a single-ended (SE) connector; SE uses 5 V logic and a single signal line driven against ground. Maximum cable length is 6 m for Slow/Fast SCSI or 3 m for Ultra SCSI.
SE is electrically incompatible with High Voltage Differential (HVD). Connecting SE and HVD devices on the same bus will cause damage. Low Voltage Differential (LVD/MSE) devices can detect the bus type via the DIFFSENSE signal and downgrade to SE mode automatically, but doing so limits the entire bus to SE speeds.
Two terminators are required per SCSI bus segment, fitted at the two physical endpoints only. Passive termination (220/330 Ω resistors) is acceptable only for Slow SCSI (5 MB/s maximum); active termination is required for Fast SCSI and above.
The female HD-50 socket is fitted to the controller and to each device; the male HD-50 plug is on the cable.
The connector uses a 1.27 mm contact pitch in a shielded D-shell, making it mechanically incompatible with the older Centronics CN-50 (2.16 mm pitch) external connector despite carrying identical electrical signals.

Signals

DB0–DB7 — 8-bit parallel data bus (narrow SCSI); active-low, all bidirectional.
PARITY — odd parity bit for DB0–DB7; bidirectional; covers the eight data lines.
/ATN — Attention; driven by the initiator to request a Message Out phase from the target.
n/c — Pin 42 is not connected; reserved.
/BSY — Busy; asserted by any device currently using the bus or during arbitration.
/ACK — Acknowledge; asserted by the initiator to handshake each byte transfer with /REQ.
/RST — Reset; asserted to reset all devices on the SCSI bus immediately.
/MSG — Message; asserted by the target during Message phases.
/SEL — Select; used during the Selection and Reselection phases to address a specific device.
/C/D — Control/Data; indicates whether the current transfer carries control information (high) or data (low).
/REQ — Request; asserted by the target to initiate each byte handshake with /ACK.
/I/O — Input/Output; indicates data transfer direction relative to the initiator (asserted = input to initiator).
TMPWR (TERMPWR) — Termination power (pin 38); supplies voltage to terminator circuits at the bus endpoints.
GND — Ground; pins 1–25 and selected higher-numbered pins are all signal-return grounds, one per signal line.

References

Category:Buses Connectors