Quantum 10K II - Manual (Page 338)

Appendix B. SCSI Bus Signal Timing
Quantum Atlas 10K II Ultra 160/m SCSI Hard Disk Drives B–17
B.5.5 Information Transfer Phases
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. 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.
Direction of Transfer
The direction of an information transfer is indicated by the I/O signal, as outlined in
Table B-8. A True I/O Signal (1) indicates a transfer from the drive to the initiator.
A False I/O Signal (0) indicates a transfer from the initiator to the drive.