1.6.2.2 Qas Arbitration - Fujitsu MBA3073 SERIES Technical Manual

1.6.2.2 QAS ARBITRATION

(1) QAS protocol
QAS allows a TARG with an information unit transfer agreement in effect and QAS enabled (see
2.3.22 that is currently connected to an INIT that has QAS enabled to transfer control of the bus to
another SCSI device that has QAS enabled without an intervening BUS FREE phase. SCSI
devices that participate in QAS arbitration shall report that capability in the INQUIRY command.
Before the INIT may use QAS that INIT shall negotiate, using the PPR message, the use of the
QAS phase with each TARG that has indicated support of QAS. Any time the INIT's negotiation
required flag is true (see 4.12) that INIT shall renegotiate to enable QAS. SCSI devices that
support QAS shall implement the fairness algorithm (see 1.13) 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, TARG shall have negotiated the use of QAS with a particular
INIT before using QAS REQUEST message to do a physical disconnect from that INIT, and INIT
shall have negotiated the use of QAS with a particular TARG before accepting a QAS REQUEST
message from that TARG. If the INIT receives a QAS REQUEST message from a TARG that has
not negotiated the use of QAS, then the INIT 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 arbitrating for the bus. This occurs when SCSI devices that have QAS enabled
never go to a BUS FREE phase.
A QAS INIT may interrupt a sequence of QAS cycles to force a normal arbitration with the
following procedure:
1) Perform a QAS arbitration;
2) On winning QAS arbitration, continue driving the INIT ID on the DATA BUS instead of
asserting SEL to enter selection phase;
3) Wait until the TARG transitions to BUS FREE (i.e., after two QAS arbitration delays);
4) After detecting BSY false, release the DATA BUS; and
5) After one bus settle delay from when the TARG drove BSY false, the bus is in BUS FREE
phase. The INIT port may then arbitrate using normal arbitration and perform a selection if it
wins.
(2) QAS phase
The procedure for a TARG with both information unit transfers and QAS enabled to Indicate it
wants to release the bus after a DT DATA phase is as follows:
1) The TARG changes to a MESSAGE IN phase and issue a single QAS REQUEST message
(see 2.3.22) and wait for ACK to be true.
2) After detection of the ACK signal being false and if the INIT did not create an attention
condition, the TARG releases all SCSI signals except the BSY, MSG, C/D, I/O, and REQ
signals. Then the TARG negates the MSG, C/D, and I/O signals within two system deskew
delays. The TARG waits two system deskew delays after negating the C/D, I/O, and MSG
signals before releasing the REQ signal.
3) If the INIT did not create an attention condition then the INIT shall release all SCSI signals
except ACK and ATN within two system deskew delays after detecting MSG, C/D, and I/O
signals false.
64 C141-C015