Quantum DLT 8000 Product Manual page 121

7. The tape drive does not act as an initiator on the SCSI bus. Therefore, the drive
does not 1) generate unsolicited interrupts to the bus, 2) initiate its own SCSI
commands, and 3) assert bus reset.
8. A mini-library subsystem is assigned two logical unit numbers (LUNs): the
tape drive is always LUN 0, and the mini-library component has a default LUN
of 1, but may be reconfigured to any LUN from 0 to 15 via the SCSI MODE
SELECT command.
The COMMAND, DATA, STATUS, and MESSAGE phases are known as the
Information Transfer Phases because they are used to transfer data or control
information.
The C/D, I/O, and MSG signals are used to distinguish between the different
information transfer phases (Table 3 5). The drive asserts these three signals and so
controls all information transfer phase changes. The drive can also cause a BUS FREE
phase by releasing the MSG, C/D, I/O, and BSY signals. The initiator can request a
MESSAGE OUT phase by asserting the ATN signal.
The information transfer phases use one or more REQ/ACK handshakes to control the
information transfer. Each REQ/ACK handshake allows the transfer of one byte of
information. During the information transfer phases, the BSY signal remains true and
the SEL signal remains false. Additionally, the drive continuously envelopes the
REQ/ACK handshake(s) with the C/D, I/O, and MSG signals in such a manner that
these control signals are valid for one bus settle delay before the assertion of the REQ
signal of the first handshake and remain valid after the negation of the ACK signal at
the end of the handshake of the last transfer of the phase.
After the negation of the ACK signal of the last transfer of the phase, the drive can
prepare for a new phase by asserting or negating the C/D, I/O, and MSG signals. These
signals can be changed together or individually. They can be changed in any order and
can be changed more than once (although each line should change only once). A new
phase does not begin until the REQ signal is asserted for the first byte of the new phase.
A phase ends when the C/D, I/O, or MSG signal changes after the negation of the ACK
signal. The time between the end of a phase and the assertion of the REQ signal
beginning a new phase is undefined. An initiator is allowed to anticipate a new phase
based on the previous phase, the expected new phase, and early information provided by
changes in the C/D, I/O, and MSG signals. However, the anticipated phase is not valid
until the REQ signal is asserted at the beginning of the next phase.
NOTES (continued)
Quantum DLT 8000 Tape System
SCSI Description
3-19