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SCSI Description
Quantum DLT 7000 Tape System 3-7
Table 3–3 SCSI-2 Bus Signal Definitions (continued)
Signal Definition
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, ACK.
RST (reset) An OR-tied signal that initiates a RESET condition.
SEL (select) An OR-tied signal used by an initiator to select a target or by a
target to reselect an initiator.
3.4.2 Signal Bus Timing
The ANSI SCSI-2 standard defines the SCSI bus timing values shown in Table
3–4.
Table 3–4 SCSI Bus Timing Values
Timing Description Value Description
Arbitration Delay 2.4 µs Minimum time a SCSI device waits from asserting BSY
for arbitration until the DATA BUS can be examined to
see if arbitration has been won; there is no maximum
time.
Assertion Period 90 ns Minimum time a drive asserts REQ while using
synchronous data transfers; also, the minimum time
that an initiator asserts ACK while using synchronous
data transfers.