Quantum 10K II - Manual (Page 334)

Appendix B. SCSI Bus Signal Timing
Quantum Atlas 10K II Ultra 160/m SCSI Hard Disk Drives B–13
2. SCSI devices release all SCSI bus signals within one bus clear delay.
If a SCSI device requires more than one bus settle delay to detect the BUS FREE
phase, then it releases all SCSI bus signals within one bus clear delay minus the
excess time to detect the BUS FREE phase.
The total time to clear the SCSI bus cannot exceed one bus settle delay plus one bus
clear delay.
B.5.2 ARBITRATION Phase
The ARBITRATION phase allows one SCSI device to gain control of the SCSI bus
so that it can initiate or resume an I/O process.
A SCSI device arbitrates for the SCSI bus by asserting both the BSY signal and its
own SCSI ID after a BUS FREE phase occurs. If a higher priority SCSI ID bit is
asserted on the bus, the lower-priority SCSI device loses the arbitration. Narrow
data bus modules/drives (8-bit) recognize SCSI Ids 7 through 0 in descending
priority in arbitration. Wide data bus modules/drives (16-bit) recognize SCSI IDs 15
through 0 in descending order (ID bit numbers 7 through 0, then 15 through 8).
Arbitration Sequence
1. The SCSI device waits for the BUS FREE phase to occur.
2. The SCSI device waits a minimum of one bus free delay after detection of the
BUS FREE phase before driving any signal.
3. The SCSI device arbitrates for the SCSI bus by asserting the BSY signal and its
SCSI ID.
4. The SCSI device waits at least an arbitration delay to determine arbitration
results:
NOTE
Step 4 requires that every device complete the arbitration phase to the
point of SEL being asserted (for a SELECTION or RESELECTION phase) to
avoid hanging the bus.
• If a higher priority SCSI ID bit is true on the DATA BUS, the SCSI device loses
the arbitration.
• The losing SCSI device releases the BSY signal and its SCSI ID bit within one
bus clear delay after the SEL signal asserted by the arbitration winner becomes
true.
• The losing SCSI device waits for the SEL signal to become true before releasing
the BSY signal and SCSI ID bit. The losing SCSI device returns to step 1
• If no higher priority SCSI ID bit is true on the DATA BUS, the SCSI device
wins the arbitration and asserts the SEL signal.
• The winning SCSI device waits at least one bus clear delay plus one bus settle
delay after asserting the SEL signal before changing any signals.