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SCSI-1 (Small Computer System Interface) Connector pin assignments

SCSI - Standard

SCSI (Small Computer System Interface) is an intelligent device interface that allows up to 8 devices (usually 7 plus the host system) to be connected together using up to a 6 meter cable. Unlike other older interfaces, requests on a SCSI bus are just that, requests. For example, rather than the low-level device commands like the familiar STEP+ and STEP- of ST412, SCSI accesses data using the higher level SEEK to a location ( LBA or logical block address on drives ).

Input voltages may vary from a low of 0VDC to a high of +5.25VDC. These devices ARE NOT* BUS COMPATIBLE with differential SCSI devices. They are hovever command compatible. Attempting to share the SCSI bus with a differential device will quite possibly damage the single-ended SCSI device. Pin designations are as follows, and all signals are active low (true is a voltage level between 0 and +0.8 VDC, and false is defined as a voltage between +2 and +5.25 VDC).

Pin number	signal	description
1	GND	Ground
2	DB0	Data bit 0	(Data Bus): Contains the data that is sent from one device to the other. Driven by either device, as determined by the state of the I/O signal.
3	GND	Ground
4	DB1	Data bit 1
5	GND	Ground
6	DB2	Data bit 2
7	GND	Ground
8	DB3	Data bit 3
9	GND	Ground
10	DB4	Data bit 4
11	GND	Ground
12	DB5	Data bit 5
13	GND	Ground
14	DB6	Data bit 6
15	GND	Ground
16	DB7	Data bit 7
17	GND	Ground
18	DBP	Data parity	Contains the parity bit for the data that is sent on DB(7-0) from one device to the other during an information transfer.
19	GND	Ground
20	GND	Ground
21	GND	Ground
22	GND	Ground
23	GND	Ground
24	GND	Ground
25	GND	Ground
26	TERMPWR	Termination pwr
27	GND	Ground
28	GND	Ground
29	GND	Ground
30	GND	Ground
31	GND	Ground
32	ATN	Attention	Driven by an Initiator to indicate the attention condition.
33	GND	Ground
34	GND	Ground
35	GND	Ground
36	BSY	Busy	"Or-tied" which indicates that the bus is in use.
37	GND	Ground
38	ACK	Acknowledge	Driven by an Initiator to indicate an acknowledgement for a REQ/ACK data transfer handshake.
39	GND	Ground
40	RST	Reset	"Or-tied" which indicates the reset condition.
41	GND	Ground
42	MSG	Message	Driven by a Target during the message phase.
43	GND	Ground
44	SEL	Select	Used by an Initiator to select a Target or by a Target to reselect an Intiator.
45	GND	Ground
46	C/D	Control/data	Driven by a Target to indicate whether control or data information is on the data bus. True indicates Control.
47	GND	Ground
48	REQ	Request	Driven by a Target to indicate a request for a REQ/ACK data transfer handshake.
49	GND	Ground
50	I/O	Input/output	Driven by a Target which controls the direction of data flow in the data bus, with respect to an Initiator. True indicates input to the Initiator.

SCSI - Differential

Differential SCSI (Small Computer System Interface) varies from standard SCSI at the bus level in that it is capable of operating up to a 25 meter cable, and the electrical specifications are quite different. Input voltages vary from a low of -7VDC to a high of +12VDC. These high voltage changes are measured from the positive (+ or POS) portion of a signal, in relation to the negative (- or NEG) signal of the same name.

These devices ARE NOT* BUS COMPATIBLE with SCSI standard devices. They are hovever command compatible. Attempting to share the SCSI bus with a non-differential device will quite possibly damage the single-ended SCSI device. The electrical specification follows the EIA RS-485-1983 standard. Pin designations are as follows:

Pin number	Signal	Description
1	GND	Shield ground
2	GND	Ground
3	+DB0	Pos data bit 0
4	-DB0	Neg data bit 0
5	+DB1	Pos data bit 1
6	-DB1	Neg data bit 1
7	+DB2	Pos data bit 2
8	-DB2	Neg data bit 2
9	+DB3	Pos data bit 3
10	-DB3	Neg data bit 3
11	+DB4	Pos data bit 4
12	-DB4	Neg data bit 4
13	+DB5	Pos data bit 5
14	-DB5	Neg data bit 5
15	+DB6	Pos data bit 6
16	-DB6	Neg data bit 6
17	+DB7	Pos data bit 7
18	-DB7	Neg data bit 7
19	+DBP	Pos parity bit
20	-DBP	Neg parity bit
21	DIFFSEN	Not used
22	GND	Ground
23	GND	Ground
24	GND	Ground
25	TERMPWR	Ground
26	TERMPWR	Termination pwr
27	GND	Ground
28	GND	Ground
29	+ATN	Pos attention
30	-ATN	Neg attention
31	GND	Ground
32	GND	Ground
33	+BSY	Pos busy
34	-BSY	Neg busy
35	+ACK	Pos acknowledge
36	-ACK	Neg acknowledge
37	+RST	Pos reset
38	-RST	Neg reset
39	+MSG	Pos message
40	-MSG	Neg message
41	+SEL	Pos select
42	-SEL	Neg select
43	+C/D	Pos command/data
44	-C/D	Neg command/data
45	+REQ	Pos request
46	-REQ	Neg request
47	+I/O	Pos input/output
48	-I/O	Neg input/output
49	GND	Ground
50	GND	Ground