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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.
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The ANSI SCSI-2 standard defines the SCSI bus timing values shown in
Table 3–4.
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.
Bus Clear Delay 800 ns Maximum time for a SCSI device to stop driving all bus
signals after:
1. BUS FREE is detected.
2. SEL is received from another SCSI device during
ARBITRATION.
3. Transition of RST to true.
For condition 1, the maximum time for a SCSI device to
clear the bus is 1200 ns (1.2 µs) from BSY and SEL first
becoming both false.
If a SCSI device requires more than a bus settle delay to
detect BUS FREE, it clears the bus within a bus clear delay
minus the excess time.
Bus Free Delay 800 ns Maximum time a SCSI device waits from its detection of
BUS FREE until its assertion of BSY when going to
ARBITRATION.
Bus Set Delay 1.8 µs Maximum time for a device to assert BSY and its SCSI ID
bit on the DATA BUS after it detects BUS FREE to enter
ARBITRATION.
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