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PRODUCT MANUAL 433689-01
CHAPTER 6 COMMUNICATION INTERFACE AND COMMAND PROTOCOL
The format and content of the information carried over the communication
interface, as well as how each device uses and responds to the information, is
governed by a command protocol. The command protocol determines how the
host (or initiator) interacts with the target device (for example, the tape drive)
by issuing commands to control its operation, transferring data, and
responding to status information. The target device responds to commands
from the host by performing the requested operation (for example, writing or
reading data on magnetic tape) and returning status information.
The VXA-320 tape drive is available with a parallel SCSI communication
interface. This communication interface provides a method of passing SCSI
command descriptor blocks (CDBs) over an bus. The operation of the tape
drive is governed by the SCSI command protocol.
The following section describes how the SCSI communication interface is
implemented in the tape drive. “SCSI Interface Requirements” on page 40
describes the physical requirements of the SCSI bus. “SCSI Command
Protocol” on page 44 provides information about the SCSI command protocol
used by the tape drive. Refer to the VXA-320 (VXA-3) Tape Drive SCSI Reference
for detailed information about the SCSI communication interface and
command protocol.
SCSI COMMUNICATION INTERFACE
This section provides an overview of the SCSI communication interface used
by the VXA-320 tape drive.
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 ble 6 - 1 describes the bus phases.
Table 6-1 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 tape drive for communication.
Reselection The Reselection phase allows the tape drive to reconnect to the initiator after it
disconnects.