SCSI-1 (SMALL COMPUTER SYSTEM INTERFACE) CONNECTOR PIN ASSIGNMENTS 50-pin Cable
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).
ACK (Acknowledge): A signal driven by an Initiator to indicate an acknowledgement for a REQ/ACK data transfer handshake
ATN (Attention): A signal driven by an Initiator to indicate the attention condition
BSY (Busy): An "or-tied" signal which indicates that the bus is in use
C/D (Control/Data): A signal driven by a Target. It indicates whether control or data information is on the data bus. True indicates Control
DB(7-0) (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
I/O (Input/Output): A signal driven by a Target which controls the direction of data flow in the data bus, with respect to an Initiator
MSG (Message): A signal driven by a Target during the message phase
REQ (Request): A signal driven by a Target to indicate a request for a REQ/ACK data transfer handshake
RST (Reset): An "or-tied" signal which indicates the reset condition
SEL (Select): A signal used by an Initiator to select a Target or by a Target to reselect an Intiator
| Pin Number | Signal | Description | Pin Number | Signal | Description |
| 1 | GND | Ground | 26 | TERMPWR | Terminator Power |
| 2 | DB0 | Data Bit 0 | 27 | GND | Ground |
| 3 | GND | Ground | 28 | GND | Ground |
| 4 | DB1 | Data Bit 1 | 29 | GND | Ground |
| 5 | GND | Ground | 30 | GND | Ground |
| 6 | DB2 | Data Bit 2 | 31 | GND | Ground |
| 7 | GND | Ground | 32 | ATN | Attention |
| 8 | DB3 | Data Bit 3 | 33 | GND | Ground |
| 9 | GND | Ground | 34 | GND | Ground |
| 10 | DB4 | Data Bit 4 | 35 | GND | Ground |
| 11 | GND | Ground | 36 | BSY | Busy |
| 12 | DB5 | Data Bit 5 | 37 | GND | Ground |
| 13 | GND | Ground | 38 | ACK | Acknowledge |
| 14 | DB6 | Data Bit 6 | 39 | GND | Ground |
| 15 | GND | Ground | 40 | RST | Reset |
| 16 | DB7 | Data Bit 7 | 41 | GND | Ground |
| 17 | GND | Ground | 42 | MSG | Message |
| 18 | DBP | Data Parity | 43 | GND | Ground |
| 19 | GND | Ground | 44 | SEL | Select |
| 20 | GND | Ground | 45 | GND | Ground |
| 21 | GND | Ground | 46 | C/D | Control Data |
| 22 | GND | Ground | 47 | GND | Ground |
| 23 | GND | Ground | 48 | REQ | Request |
| 24 | GND | Ground | 49 | GND | Ground |
| 25 | GND | Ground | 50 | I/O | Input/Output |