Small Computer System Interface (SCSI) Connector Pin Assignments and Signals

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Single-Ended SCSI Connector Pin Assignments and Signals

The Small Computer System Interface, or SCSI, is a standard interface and command set for transferring data between devices connected to both internal and external buses of a computer system. Pronounced as "scuzzy", SCSI is most commonly used for storage devices such as hard disks and tape back-up, but it is also used to connect a wide variety of other devices, including scanners, printers, CD-ROM drives, CD recorders, and DVD drives. Other technologies, like the serial-ATA (SATA), have largely replaced SCSI in new systems.

SCSI may either be a 'single-ended' data bus transmitting data referenced to the ground (unbalanced), or it can be a 'differential' data bus (balanced). Single-ended and differential SCSI devices can not be mixed on the same physical bus since they are electrically incompatible. Table 1 below shows the signals employed by a single-ended 50-pin SCSI.

Table 1. Single-Ended 50-pin SCSI Bus Signals

Pin #	SCSI Signal	Direction	Pin #	SCSI Signal	Direction
1	D0 Ground	I/O	26	Spare	-
2	D0	I/O	27	Ground	-
3	D1 Ground	I/O	28	Spare	-
4	D1	I/O	29	Ground	-
5	D2 Ground	I/O	30	Spare	-
6	D2	I/O	31	Ground	-
7	D3 Ground	I/O	32	Attention	O
8	D3	I/O	33	Attention Ground	O
9	D4 Ground	I/O	34	Spare	-
10	D4	I/O	35	Busy Ground	O
11	D5 Ground	I/O	36	Busy	O
12	D5	I/O	37	ACK Ground	I
13	D6 Ground	I/O	38	Acknowledge (ACK)	I
14	D6	I/O	39	RST Ground	I
15	D7 Ground	I/O	40	Reset (RST)	I
16	D7	I/O	41	MSG Ground	O
17	Parity/Ground	I	42	Message (MSG)	O
18	Parity	I	43	SEL Ground	I
19	Ground	-	44	Select (SEL)	I
20	Spare	-	45	C/D Ground	O
21	Ground	-	46	Command/Data (C/D)	O
22	Spare	-	47	REQ Ground	O
23	Ground	-	48	Request (REQ)	O
24	Spare	-	49	I/O Ground	I
25	Ground	-	50	I/O	I/O