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3.9.2.1
MESSAGE IN phase exception condition handling
If the SCSI initiator port detects a parity error on any message byte it receives, the SCSI initiator port shall cre-
ate an attention condition. When the SCSI target port switches to a MESSAGE OUT phase, the SCSI initiator
port shall send a MESSAGE PARITY ERROR message (see Section 4.3.7) to the SCSI target port. This mes-
sage notifies the SCSI target port that the MESSAGE IN byte was invalid.
3.9.3
MESSAGE OUT phase
The MESSAGE OUT phase allows the SCSI target port to request that message(s) be sent from the initiator to
the target. The target invokes this phase in response to the attention condition created by the initiator (see Sec-
tion 5.1).
The target shall assert the C/D and MSG signals and negate the I/O signal during the REQ/ACK handshakes
of this phase. The target shall handshake bytes in this phase until the attention condition is cleared, except
when rejecting a message.
A QAS-capable initiator shall wait a minimum of one QAS non-data phase REQ(ACK) period to assert ACK
after detecting the assertion of REQ.
A QAS-capable initiator shall assert ACK for a minimum of one QAS non-data phase REQ(ACK) period and
shall keep the message data valid until the negation of ACK.
If the target receives all of the message bytes successfully (i.e. no parity errors), it shall indicate that no retry is
being attempted by changing to any information transfer phase other than the MESSAGE OUT phase and
transferring at least one byte. The target may also indicate that it has successfully received the message bytes
by changing to the BUS FREE phase (e.g., after receiving ABORT TASK SET or TARGET RESET messages).
3.9.3.1
MESSAGE OUT phase exception condition handling
If the target detects one or more parity errors on the message bytes received, it may request a retry of the mes-
sages by asserting the REQ signal after detecting the attention condition has been cleared and prior to chang-
ing to any other phase. The initiator, upon detecting this condition, shall resend all of the previous message
bytes in the same order as previously sent during this phase. When resending more than one message byte,
the initiator shall reestablish the attention condition as described in Section 5.1.
If the target does not retry the MESSAGE OUT phase or it exhausts its retry limit, it may:
a. return CHECK CONDITION status and set the sense key to Aborted Command and the additional sense
code to Message Error; or
b. indicate a protocol error by performing an unexpected bus free.
The target may act on messages as received as long as no parity error is detected and may ignore all remain-
ing messages sent under one attention condition after a parity error is detected. When a sequence of mes-
sages is resent by a SCSI initiator port because of a SCSI target port detected parity error, the target shall not
act on any message that it acted on the first time received.
3.10
Signal restrictions between phases
When the SCSI bus is between two information transfer phases, the following restrictions shall apply to the
SCSI bus signals:
a. The BSY, SEL, and ACK signals shall not change.
b. The REQ signal shall not change until it is asserted to qualify the start of a new phase.
c. The C/D, I/O, MSG, Data Bus, and DB(P_CRCA) signals may change.
d. When switching the Data Bus or DB(P_CRCA) signal direction from out (SCSI initiator port driving) to in
(SCSI target port driving), the target shall delay driving the Data Bus, DB(P_CRCA), and/or DB(P1) by at
least one data release delay plus one bus settle delay after asserting the I/O signal, and the initiator shall
release the Data Bus, DB(P_CRCA), and/or DB(P1) no later than one data release delay after the transition
of the I/O signal to true. When switching the Data Bus, DB(P_CRCA), and/or DB(P1) direction from in (tar-
get driving) to out (initiator driving), the target shall release the Data Bus, DB(P_CRCA), and/or DB(P1) no
later than one system deskew delay after negating the I/O signal. The initiator shall assert the Data Bus,
SCSI Interface Product Manual, Rev. B
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