Tandberg Data VXA-320 (VXA-3) - Manual (Page 53)

O CTOBER 2006 VXA-320 T APE D RIVE 39
SCSI C
OMMUNICATION
I
NTERFACE
SCSI M
ESSAGE
S
YSTEM
The SCSI message system allows communication between a SCSI initiator and
the VXA-320 tape drive for interface management. This message system or
command set is separate from the SCSI command protocol used to control the
operation of the tape drive. A message can be one byte or multiple bytes.
Table 6-2 describes the SCSI messages that the tape drive supports.
Transfer:
ƒ
Message In/
Message Out
ƒ Command
Out
ƒ Data In/
Data Out
ƒ Status In
The Message phases help manage the physical path between the initiators and targets.
ƒ
In the Message In phase, the tape drive sends a message to the initiator. In the Message
Out phase, the initiator sends a message to the tape drive.
ƒ In the Command Out phase, the initiator sends a command to the tape drive.
Commands contain information about what actions the tape drive should perform.
ƒ In the Data In phase, the tape drive transfers data to the initiator. In the Data Out
phase, the initiator transfers data to the tape drive.
ƒ In the Status In phase, the tape drive returns a status byte to the initiator. The status
byte indicates the results of the command’s execution.
Table 6-1 SCSI bus phases and information transfer phases
Bus Phase Description
Table 6-2 SCSI messages supported by the tape drive
Message
Hex
Code
Description
Task
Complete
00h The tape drive informs the initiator that the execution of the command was
completed and that it sent a valid status byte to the initiator.
Extended
Messages
01h Synchronous Data Transfer Request (01h) The tape drive supports
synchronous data transfer.
Wide Data Transfer Request (03h)
The tape drive supports wide data transfer.
Parallel Protocol Request (04h) The tape drive supports negotiating a
synchronous data transfer agreement, a wide data transfer agreement, and setting
the protocol options between two SCSI devices.
Save Data
Pointer
02h The tape drive requests that the initiator copy the current data pointer for possible
use by the Restore Pointers message during error recovery.
Restore
Pointers
03h The tape drive informs the initiator that it did not properly receive a block of data
or the command descriptor block (CDB) and that the data needs to be transferred
again.
Disconnect 04h The tape drive informs the initiator that it plans to disconnect from the SCSI bus
and that a reconnect will be required later.
Initiator
Detected
Error
05h The initiator informs the tape drive that an error occurred. The tape drive can retry
the operation.