Status Phase; Message Phase; Message In Phase; Message Out Phase - Seagate SCSI Interface Product Manual

SCSI Interface Product Manual, Rev. J
3.1.8

Status phase

The Status phase allows the target to request that it send status information to the initiator.
See Section 4.3 for details.
The target shall assert C/D and I/O and negate the MSG signal during the REQ/ACK handshake of this phase.
3.1.9

Message phase

The Message phase is a term that references either a Message In or a Message Out phase. Multiple messages
may be sent during either phase. Multiple byte messages shall be wholly contained with a single message
phase. Messages supported by a particular drive are listed in the individual drive's Product Manual, Volume 1,
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 Section 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 tar-
get shall handshake byte(s) in this phase until ATN goes false, unless an error occurs (see Message Reject, in
Section 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 asserting ACK
on the first byte and shall maintain ATN asserted until the last byte is sent as described in Section 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 changing 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 direc-
tion 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 (see
Section 3.2.1) and Reset condition (see Section 3.2.2).
3.2

SCSI bus asynchronous conditions

The SCSI bus has two asynchronous conditions: the Attention condition and the Reset condition. These condi-
tions cause the SCSI device to perform certain actions and can alter the phase sequence.
3.2.1

Attention condition

The Attention condition allows an initiator to inform a target that the initiator has a message ready. The target
gets this message at its convenience by performing a Message Out phase.
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