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ST43401N/ND and ST43402ND Reference Manual, Rev. C
The selected device responds to the initiator by entering the Message Out
phase if the initiator has Attention asserted. In this phase, the target
requests that the initiator send messages to it. The Message, Control/
Data, and Input/Output lines are used in combinations to indicate the
various information transfer phases. The state of the three signal lines is
controlled by the target, and the phase selected by the state of these three
signal lines is shown earlier in Table 7.
In this example, the Identify message is the first message sent by the initiator
after the Selection phase. This message identifies the physical path for the
logical unit (only logical unit 0 is supported) specified by the initiator. The
Identify message is also the first message sent by the target following the
Reselection phase. The initiator can request a Message Out phase by
asserting Attention.
Following the Message Out phase, the initiator responds to the Command
phase and, in this example, issues a Read command to the drive. The
Message In phase is entered (Disconnect message) followed by the Bus Free
phase. The Arbitration phase is again entered, followed by the Reselection
and the Message In (Identify message) phases. Reselection is a phase that
allows the target to reconnect to the initiator so it can continue an operation
that was started by an initiator but suspended by the target before it was
complete. The requested read data is then transferred to the initiator.
Following the Data In phase, the target enters the Message In phase and, in
this example, requests the initiator to Save Data Pointer. There are current
(also called active) pointers that represent the state of the interface and point
to the next command, data, or status byte to be transferred between the
initiator's memory and the target. Current pointers are used with the target
currently connected to the initiator.
Another set of pointers called saved pointers is provided for each active
command, whether or not it is currently connected. The command pointer
points to the start of the command descriptor block for that command. The
saved data pointer points to the start of the data area at the beginning of each
command and it remains at this value until the target sends the Save Data
Pointer message to the initiator. In response to this message, the initiator
stores the value of the current data pointer into the saved data pointer. Only
the saved pointer values are retained when a device disconnects from the
bus. The current pointer values are restored from the saved values at the next
reconnection.
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