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The following paragraphs define SCSI signals and bus timing values.

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Table 3–3 defines the SCSI bus signals.

ACK (acknowledge) A signal driven by the initiator as an acknowledgment of receipt of
data from a target or as a signal to a target indicating when the
target should read the data (out) lines.
ATN (attention) A signal driven by an initiator to indicate that it has a message to
send.
BSY (busy) An OR-tied signal that indicates that the bus is in use.
C/D (control/data) A signal driven by a target that indicates whether CONTROL or
DATA information is on the DATA BUS. True (low voltage)
indicates CONTROL.
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 (8
or 16-bit) during ARBITRATION. Bit number, significance, and
priority decrease downward to DB(0). A data bit is defined as 1
when the signal value is true (low voltage) and 0 when the signal
value is false (high voltage). Data parity DB(P) is odd. Parity is
undefined during ARBITRATION.
DB(15–8,P1) (data bus) Eight data-bit signals, plus one parity-bit signal, that forms an
extension to the DATA BUS. They are used for 16-bit (wide)
interfaces. DB(15) is the most significant bit and has the higher
priority (but below bit DB(0) during ARBITRATION. Bit number,
significance, and priority decrease downward to DB(8). Data Parity
DB (P1) is odd.
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.
Also used to distinguish between SELECTION and RESELECTION
modes.
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 an information
transfer to or from the initiator. Each byte of data transferred is
accompanied with a REQ/ACK “handshake”. See also,
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