Qas Protocol - Seagate Ultra160 Product Manual

SCSI Interface Product Manual, Rev. B
1. 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.
2. 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
(a). If the SCSI device implements a "fairness algorithm" for arbitration, see Section 3.4.
Note. Step (d) 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 (c). This precludes the possibility of the bus being hung.
e. After the bus free delay in step (b), 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 (b). 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.
f. 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.
3.1.2.2

QAS protocol

Quick Arbitration and Selection (QAS) allows a SCSI target port with an information unit transfer agreement in
effect and QAS enabled (see Section 4.3.12) that is currently connected to a SCSI initiator port that has infor-
mation unit transfers enabled and QAS enabled to transfer control of the bus to another SCSI device that has
information unit transfers enabled and QAS enabled without an intervening BUS FREE phase. SCSI devices
that support QAS shall report that capability in the INQUIRY command.
Before a SCSI initiator may use QAS, that initiator shall negotiate, using the PPR message, the use of the QAS
phase with each SCSI target port that has indicated support of QAS. Any time a SCSI initiator port's negotia-
tion required flag is true, that SCSI initiator port shall renegotiate to enable QAS (see Section 4.3.12).
SCSI devices that support QAS shall implement the fairness algorithm (see Annex B of SPI-4) during all QAS
arbitrations. SCSI devices shall negotiate the use of QAS with a particular SCSI device before using QAS to
select or reselect that SCSI device. Also, targets shall have negotiated the use of QAS with a particular initiator
before using QAS REQUEST message to do a physical disconnect from that initiator, and initiators shall have
negotiated the use of QAS with a particular target before accepting a QAS REQUEST message from that tar-
get. If a SCSI initiator port receives a QAS REQUEST message from a SCSI target port that has not negoti-
ated the use of QAS, then the initiator shall create an attention condition for the QAS REQUEST message, and
shall report Message Reject on the following MESSAGE OUT phase.
In an environment where some SCSI devices have QAS enabled and other SCSI devices do not, it is possible
for the SCSI devices that have QAS enabled to prevent SCSI devices that do not have QAS enabled from arbi-
trating for the bus. This occurs when SCSI devices that have QAS enabled never go to a BUS FREE phase.
A QAS initiator may interrupt a sequence of QAS cycles to force a normal arbitration with the following proce-
dure:
1. perform a QAS arbitration;
2. on winning QAS arbitration, continue driving the initiator's ID on the Data Bus instead of asserting SEL to
enter selection phase;
3. wait until the target transitions to Bus Free (this occurs after two QAS arbitration delays);
4. after detecting BSY false, release the Data Bus; and
5. after one bus settle delay from when the target drove BSY false, the bus is in BUS FREE phase. The initia-
tor may then arbitrate using normal arbitration and perform a selection if it wins.
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