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46
m. after a PPR (Parallel Protocol Request) negotiation in response to a selection using attention condition
when information unit transfers are enabled (see Section 4.3.12); or
n. after any successful negotiation that causes information unit transfers to be enabled or disabled.
The target uses an unexpected bus free to inform the initiator of a protocol error. The target may switch to a
BUS FREE phase at any time, except during an ARBITRATION phase, independent of any attention condition.
The target shall terminate the task that was the current task before the BUS FREE phase by clearing all data
and status for that task. The target may optionally prepare sense data that may be retrieved by a REQUEST
SENSE command. However, an unexpected bus free shall not create an exception condition.
The initiator shall terminate the task that was the current task before the BUS FREE phase occurred and shall
manage this condition as an exception condition.
3.1.1.2
Expected bus free phases
Initiators may expect a bus free to occur after one of the following:
a. after the last SPI command information unit is successfully received by a SCSI target port;
b. after a SPI data information unit is successful received by or transmitted from a SCSI target port;
c. after a SPI status information unit is successfully transmitted from a SCSI target port;
d. after a SPI L_Q information unit, if the SPI L_Q information unit Data Length field is zero; or
e. during a QAS phase.
3.1.2
Arbitration and QAS overview
Arbitration allows one SCSI device to gain control of the SCSI bus so that it can initiate or resume a task.
There are two methods that a SCSI device may use to arbitrate for the SCSI bus: normal arbitration and QAS
(Quick Arbitration and Selection). Normal arbitration is mandatory and requires the detection of a BUS FREE
phase on the SCSI bus before starting. QAS is optional and, when enabled, requires the detection of a QAS
REQUEST message before starting.
SCSI devices with arbitration fairness enabled shall maintain a fairness register that records the SCSI IDs of
devices that need a chance to arbitrate (see Section 3.4). Fairness in normal arbitration is enabled in targets by
the Disconnect-Reconnect mode page (see Section 8.12.4). Fairness is always enabled when QAS is enabled.
3.1.2.1
Normal ARBITRATION phase
The procedure for a SCSI device to obtain control of the SCSI bus is as follows:
a. The SCSI device shall first wait for the BUS FREE phase to occur. The BUS FREE phase is detected when-
ever both the BSY and SEL signals are simultaneously and continuously false for a minimum of one bus
settle delay.
Note.
This bus settle delay is necessary because a transmission line phenomenon known as a wired-OR
glitch may cause the BSY signal to briefly appear false, even though it is being driven true.
b. The SCSI device shall wait a minimum of one bus free delay after detection of the BUS FREE phase (i.e.,
after the BSY and SEL signals are both false for one bus settle delay) before driving any signal.
c. Following the bus free delay in step (b), the SCSI device may arbitrate for the SCSI bus by asserting both
the BSY signal and its own SCSI ID. However the SCSI device shall not arbitrate (i.e., assert the BSY signal
and its SCSI ID) if more than one bus set delay has passed since the BUS FREE phase was last observed.
If arbitration fairness is enabled, the SCSI device shall not arbitrate until its fairness register is cleared (see
ANSI specification SPI-4, Annex B).
Note.
There is no maximum delay before asserting the BSY signal and the SCSI ID following the bus free
delay in step (b) as long as the bus remains in the BUS FREE phase. However, SCSI devices that
delay longer than one bus settle delay plus one bus set delay from the time when the BSY and SEL
signals first become false may fail to participate in arbitration when competing with faster SCSI
devices, and may not be ensured fair arbitration by the arbitration fairness algorithm.
d. After waiting at least one arbitration delay, measured from its assertion of BSY, the SCSI device shall exam-
ine the Data Bus.
SCSI Interface Product Manual, Rev. B
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