Operating instructions

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SCSI REFERENCE PRELIMINARY 1012975
CHAPTER 2—OVERVIEW OF THE AUTOLOADER AS A SCSI DEVICE
2.3 COMMUNICATION ACROSS THE SCSI BUS
This section explains how communication across the SCSI bus is implemented.
It discusses the SCSI bus phases and messages supported by the autoloader.
2.3.1 SCSI BUS PHASES
Bus phases determine the direction and type of information transferred across
the data lines of the SCSI bus. The possible bus phases include Bus Free,
Arbitration, Selection, Reselection, and Transfer (which includes four subsets:
Message In or Message Out, Command Out, Data In or Data Out, and Status
In). Ta b l e 2 - 1 describes the bus phases.
2.3.2 SCSI MESSAGES
The SCSI message system allows communication between an initiator and the
autoloader for physical path management. Messages allow the initiator and
the autoloader to manage error detection, data transfer retries, and the data
path. The autoloader supports the SCSI messages listed in Ta b l e 2 - 2 .
Notes:
 One or more messages can be sent during a single message phase.
 The messages supported by the autoloader are independent of the messages
supported by the tape drive. For information about the messages for the tape
drive, refer to the documentation for the tape drive.
Table 2-1 SCSI bus phases and information transfer phases
Bus Phase Description
Bus Free The Bus Free phase specifies that no device is using the bus.
Arbitration The Arbitration phase allows devices to compete for access on the bus.
Selection The Selection phase allows an initiator to select the autoloader for communication.
Reselection The Reselection phase allows the autoloader to reconnect to the initiator after it
disconnects.
Transfer:
 Message In/
Message Out
 Command
Out
 Data In/
Data Out
 Status In
The Message phases help manage the physical path between the initiators and targets.
 In the Message In phase, the autoloader sends a message to the initiator. In the
Message Out phase, the initiator sends a message to the autoloader.
 In the Command Out phase, the initiator sends a command to the autoloader.
Commands contain information about what actions the autoloader should perform.
 In the Data In phase, the autoloader transfers data to the initiator. In the Data Out
phase, the initiator transfers data to the autoloader.
 In the Status In phase, the autoloader returns a status byte to the initiator. The status
byte indicates the results of the command’s execution.