Qas Phase Overview - Seagate Ultra160 Product Manual

48
3.1.2.3

QAS phase overview

For targets with information unit transfers and QAS enabled to indicate it wants to release the bus, the following
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 condi-
tion, the SCSI target port shall release all SCSI signals except the BSY, MSG, C/D, I/O, and REQ signals.
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-4 specification.
4. If the SCSI initiator creates an attention condition, the SCSI target port shall go to a MESSAGE OUT
phase, receive all the message bytes, and cause an unexpected bus free by generating a BUS FREE
phase (see Section 3.1.1.1 on page 45).
5. If the SCSI target port detects the SEL signal being true, the SCSI target port shall release the BSY, MSG,
C/D, and I/O signals within one QAS release delay.
6. After waiting at least one QAS arbitration delay from negating the SCSI MSG, C/D, and I/O signals in step
2, if there are no SCSI ID bits true, the SCSI target port shall transition to the BUS FREE phase.
7. After waiting at least one QAS arbitration delay from negating the MSG, C/D, and I/O signals in step 2, if
there are any SCSI ID bits true, the SCSI target port shall wait at least a second QAS arbitration delay. If
the SEL signal is not true by the end of the second QAS arbitration delay, the SCSI target port shall transi-
tion to the BUS FREE phase.
Note. The release of MSG, C/D, and I/O may cause release glitches. Step 5 above ensures these glitches
occur at a time when no connection is established on the bus so that they do not interfere with proper
operation.
The procedure for a SCSI device with QAS enabled to obtain control of the SCSI bus via QAS is as follows:
1. The SCSI device shall first wait for MESSAGE IN phase to occur with a single QAS REQUEST message.
When the SCSI device detects the ACK signal being false for the QAS REQUEST message and the atten-
tion condition is cleared, it shall begin the QAS phase.
2. The SCSI device shall wait a minimum of a two system deskew delays after detection of the MSG, C/D,
and I/O signals being false before driving any signal.
3. Following the delay in step 2, the SCSI device may arbitrate for the SCSI bus by asserting its own SCSI ID
within one QAS assertion delay from detection of the MSG, C/D, and I/O signals being false. If arbitration
fairness is enabled, the SCSI device shall not arbitrate until its fairness register is cleared.
4. After waiting at least one QAS arbitration delay, measured from the detection of the MSG, C/D, and I/O sig-
nals being negated, the SCSI device shall examine the Data Bus.
a. If no higher priority SCSI ID bit is true on the Data Bus and the fairness algorithm allowed the SCSI
device to participate, then the SCSI device has won the arbitration and it shall assert the SEL signal.
b. If a higher priority SCSI ID bit is true on the Data Bus (see Table 1 for the SCSI ID arbitration priorities)
or the fairness algorithm (see Section 3.4) prevented the SCSI device from participating in QAS arbi-
tration, then the SCSI device has lost the arbitration.
c. Any SCSI device other than the winner has lost the arbitration and shall release its SCSI ID bit after
two system deskew delays and within one QAS release delay after detection of the SEL signal being
asserted. A SCSI device that loses arbitration may return to step 1.
5. The SCSI device that wins arbitration shall wait at least a QAS arbitration delay after asserting the SEL sig-
nal before changing any signals.
SCSI Interface Product Manual, Rev. B