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CDU-541 CD-ROM DRIVE SCSI INTERFACE MANUAL
4.4. Handling of 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.
The
first byte transferred in either of these phases shall. be either a single-byte
message or the first byte of a multiple-byte message.
Multiple-byte messages
shall be wholly contained within a single message phase.
4 .4 .
1.
MESSAGE
IN
Phase
The MESSAGE IN phase allows the target to request that message(s) be sent to
the initiator from the controller.
The controller will assert C/O, I/O, and MSG during the REQ/ACK handshake(s)
of this phase.
4.4.2. MESSAGE OUT Phase
The MESSAGE OUT phase allows the controller to request that message(s) be
sent from the initiator to the controller.
The controller may invoke this
phase at its convenience in response to the ATTENTION condition created by the
initiator.
The controller will assert C/O and MSG and negate I/O during the REQ/ACK
handshake(s) of this phase.
The controller will handshake byte(s) in this
phase until ATN goes false, unless an error occurs.
If the controller 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 prior to changing to any other phase.
-~-
The initiator, upon detecting this condition, shall re-send all of the
previous message byte(s) in the same order as previously sent during this
phase.
When re-sending more than one message byte, the initiator shall assert
ATN prior to asserting ACK on the first byte and shall maintain ATN asserted
until the last byte is sent.
The controller will act on messages as received as long as no parity error
is detected and will ignore all remaining messages sent under one ATN
condition after a parity error is detected.
When a sequence of messages is
re-sent by an initiator because of a target detected parity error, the target
will not act on any message which it acted on the first time received.
If the target receives all of the message byte(s) successfully (i.e., no·
parity errors), it will indicate that it does not wish to 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
4-6
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