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
3.1.2.1Normal 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.
Note. This bus settle delay is necessary because a transmission line phenomenon known as a
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
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.
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.
(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.
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.
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
Note. For ease of implementation, this sampling may begin when BSY is true following Bus Free and end 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.
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