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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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