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APPENDIX A: SCSI Signal Descriptions
A total of 18 signals are required for the SCSI interface. These signals are
described as follows:
BSY (BUSY)—
An “OR-tied” signal indicating that the bus is being used.
SEL (SELECT)—
An “OR-tied” signal used by an initiator to select a target
or by a target to reselect an initiator.
C/D (CONTROL/DATA)—
A signal driven by a target that indicates
whether CONTROL or DATA information is on the DATA BUS. True
indicates CONTROL.
I/O (INPUT/OUTPUT)—
A signal driven by a target that controls the
direction of data movement on the DATA BUS with respect to an initiator.
True indicates input to the initiator. This signal is also used to distinguish
between SELECTION and RESELECTION phases.
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.
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.
RST (RESET)—
An “OR-tied” signal that indicates the RESET condition.
DB(7-0,P) (DATA BUS)—
Eight data-bit signals, plus a parity-bit signal that
form a DATA BUS. DB(7) is the most significant bit and has the highest
priority during the ARBITRATION phase. Bit number, significance, and
priority decrease downward to DB(0). A data bit is defined as one when the
signal value is true and is defined as zero when the signal value is false. Data
parity DB(P) shall be odd.
Appendix A—SCSI Signal Descriptions
