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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-5) 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 negotiated
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 ini-
tiator may then arbitrate using normal arbitration and perform a selection if it wins.
3.1.2.3
QAS phase overview
For targets with both information unit transfers and QAS enabled to indicate it wants to release the bus, the fol-
lowing procedure is used:
1. The target shall change to a MESSAGE IN phase, issue a single QAS Request (55h) message, and
then wait for ACK to be true.
Note.
The timing requirements are required to ensure that all the SCSI devices that have QAS enabled see
the message bytes.
2. After detection of the ACK signal being false and if the SCSI initiator port did not create an attention con-
dition, the SCSI target port shall release all SCSI signals except the BSY, MSG, C/D, I/O, and REQ sig-
nals. Then the SCSI target shall negate the MSG, C/D, and I/O signals within two system deskew
delays. The SCSI target port shall wait two system deskew delays after negating the C/D, I/O, and MSG
signals before releasing the REQ signal.
3. If the SCSI initiator port did not create an attention condition, the SCSI initiator port shall release all
SCSI signals except ACK and ATN within two system deskew delays after detecting MSG, C/D, and I/O
signals false. The ACK and ATN signals shall follow the timing specified in section 7 of the SPI-5 speci-
fication.
54
Parallel SCSI Interface Product Manual, Rev. A
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