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

50-pin Cable

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

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. 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).

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

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