Product Manual - Disc Drive SCSI-2/SCSI-3 Interface (Vol. 2; Ver. 2), Rev. E
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3.1.9
Message phase
The Message phase is a term that references either a Message In or a Message Out phase. Multiple mes-
sages may be sent during either phase. Multiple byte messages shall be wholly contained with a single
message phase. Messages supported by a particular disc drive are listed in the Product Manual for that
drive.
3.1.9.1
Message in phase
The Message In phase allows the target to request that it send message(s) to the initiator.
The target shall assert C/D, I/O, and MSG during the REQ/ACK handshake(s) of this phase.
3.1.9.2
Message out phase
The Message Out phase allows the target to request that message(s) be sent from the initiator to the target.
The target may invoke this phase at its convenience in response to the Attention condition (see 3.2.1) created
by the initiator.
The target shall assert C/D and MSG and negate I/O during the REQ/ACK handshake(s) of this phase. The
target shall handshake byte(s) in this phase until ATN goes false, unless an error occurs (see Message
Reject, 3.5.2).
If the target detects one or more parity error(s) on the message byte(s) received, it may indicate its desire to
retry the message(s) by asserting REQ after detecting ATN has gone false and before changing to any other
phase. The initiator, upon detecting this condition, shall resend all of the previous message byte(s) sent
during this phase. When resending more than one message byte, the initiator shall assert ATN before assert-
ing ACK on the first byte and shall maintain ATN asserted until the last byte is sent as described in 3.2.1.
If the target receives all of the message byte(s) successfully (i.e., no parity errors), it shall indicate that it shall
not retry by changing to any information transfer phase other than the Message Out phase and transfer at
least one byte. The target may also indicate that it has successfully received the message byte(s) by chang-
ing to the Bus Free phase (e.g., Abort or Bus Device Reset messages).
3.1.10
Signal restrictions between phases
When the SCSI bus is between two information transfer phases, the following restrictions shall apply to the
SCSI bus signals:
1.
The BSY, SEL, REQ, and ACK signals shall not change.
2.
The C/D, I/O, MSG, and Data Bus signals may change. When switching the Data Bus direction from Out
(initiator driving) to In (target driving), the target shall delay driving the Data Bus by at least a data release
delay plus settle delay after asserting the I/O signal and the initiator shall release the Data Bus no later
than a data release delay after the transition of the I/O signal to true. When switching the Data Bus
direction from In (target driving) to Out (initiator driving), the target shall release the Data Bus no later
than a deskew delay after negating the I/O signal.
3.
The ATN and RST signals may change as defined under the descriptions for the Attention condition
(3.2.1) and Reset condition (3.2.2).
