Seagate Ultra 320 ユーザーズマニュアル
Parallel SCSI Interface Product Manual, Rev. A
53
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 Seagate SCSI Command Reference Manual, Part number
100293068). Fairness is always enabled when QAS is enabled.
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 Seagate SCSI Command Reference Manual, Part number
100293068). 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:
1. The SCSI device shall first wait for the BUS FREE phase to occur. The BUS FREE phase is detected
whenever both the BSY and SEL signals are simultaneously and continuously false for a minimum of
one bus settle delay.
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.
glitch may cause the BSY signal to briefly appear false, even though it is being driven true.
2. 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.
3. Following the bus free delay in step 2, 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) during this NORMAL ARBITRATION phase 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-5, Annex B).
BSY signal and its SCSI ID) during this NORMAL ARBITRATION phase 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-5, Annex B).
Note.
There is no maximum delay before asserting the BSY signal and the SCSI ID following the bus free
delay in step 2 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.
delay in step 2 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.
4. After waiting at least one arbitration delay, measured from its assertion of BSY, the SCSI device shall
examine the Data Bus.
(a) If no higher priority SCSI ID bit is true on the Data Bus [DB(7) is the highest], the SCSI device has
won the arbitration and the SCSI device shall assert the SEL signal.
won the arbitration and the SCSI device shall assert the SEL signal.
(b) If a higher priority SCSI ID bit is true on the Data Bus, the SCSI device has lost the arbitration and
it shall release the BSY signal and the SCSI ID after the SEL signal becomes true (asserted), within
one bus clear delay after the SEL signal becomes true. A SCSI device that loses arbitration may return
to step (1). If the SCSI device implements a “fairness algorithm” for arbitration, see Section 3.4.
it shall release the BSY signal and the SCSI ID after the SEL signal becomes true (asserted), within
one bus clear delay after the SEL signal becomes true. A SCSI device that loses arbitration may return
to step (1). If the SCSI device implements a “fairness algorithm” for arbitration, see Section 3.4.
Note.
Step 4 above requires any device that begins normal ARBITRATION phase to complete the normal
ARBITRATION phase to the point of SEL being asserted if it begins the normal ARBITRATION phase
as stated in step 3. This precludes the possibility of the bus being hung.
ARBITRATION phase to the point of SEL being asserted if it begins the normal ARBITRATION phase
as stated in step 3. This precludes the possibility of the bus being hung.
5. After the bus free delay in step 2, SCSI devices with arbitration fairness enabled that are not arbitrating
shall wait one bus set delay and start sampling the Data Bus to determine the SCSI devices that
attempted arbitration, the SCSI device that won, and the SCSI devices that lost. This sampling shall
continue for an arbitration delay after the bus free delay in step 2. Each SCSI device shall update its
fairness register with all lower-priority device IDs that lost arbitration.
attempted arbitration, the SCSI device that won, and the SCSI devices that lost. This sampling shall
continue for an arbitration delay after the bus free delay in step 2. Each SCSI device shall update its
fairness register with all lower-priority device IDs that lost arbitration.
Note.
For ease of implementation, this sampling may begin when BSY is true following Bus Free and end
when SEL is true.
when SEL is true.
6. The SCSI device that wins arbitration shall wait at least a bus clear delay plus a bus settle delay after
asserting SEL before changing any signals.
The SCSI ID bit is a single bit on the Data Bus that corresponds to the SCSI device’s unique SCSI address. All
other of the Data Bus bits shall be released by the SCSI device. During the normal ARBITRATION phase,
DB(P_CRCA) and DB(P1) (if present) may be released or asserted, but shall not be actively driven false.
other of the Data Bus bits shall be released by the SCSI device. During the normal ARBITRATION phase,
DB(P_CRCA) and DB(P1) (if present) may be released or asserted, but shall not be actively driven false.