Fujitsu MBA3300 NP SERIES, Technical Manual

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1-79 C141-C007
Whenever a requirement for arbitration arises, a SCSI device shall first check to see if its fairness
register is clear. If the fairness register is clear, this SCSI device may now participate in arbitration.
If the fairness register is not clear, the SCSI device must put off arbitration until all lower priority
SCSI IDs have been cleared from the fairness register.
Lower SCSI IDs are cleared as SCSI devices either win arbitration or discontinues arbitration (e.g.,
as a result of an ABORT TASK, an ABORT TASK SET, a CLEAR TASK SET, a TARGET
RESET Message and Hard Reset).
Arbitration fairness in targets is controlled with the disconnect-reconnect mode page.
(2) Determining fairness by monitoring prior bus activity
This standard requires that the SCSI IDs of all arbitrating SCSI devices appear on the bus within a
Bus Set Delay since BSY was first asserted. After this time, a SCSI device examines the bus to
detect arbitrating situation.
Since the lower priority SCSI IDs begin to disappear after an Arbitration Delay from the assertion
of BSY, the data bus shall be sampled after a Bus Set Delay but before an Arbitration Delay.
(3) Fairness states
A SCSI device is in one of three following fairness states.
•
A SCSI device is in Fairness Wait state when it is waiting for a clear fairness register to
participate in arbitration.
•
A SCSI device is in the Fairness Participate state when it is participating in arbitration.
•
A SCSI device is in the Fairness Idle state for all other conditions. A SCSI device should
enter the fairness idle state after any reset event.
A SCSI device should implement a lockout delay to prevent devices that stop arbitrating from
causing deadlock.