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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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