Quantum DLT 4000 Product Manual page 172

DLT4000 SCSI Interface
directly to the BUS FREE state by releasing the BSY signal, signifying a catastrophic
error.
The target's response to this message is to switch to the MESSAGE IN phase and re-
send from the beginning all the bytes of the message which generated the original
MESSAGE PARITY ERROR message.
MESSAGE REJECT (07h)
This message is sent from the initiator or target to indicate that the last message
received was inappropriate or has not been implemented.
To indicate its intentions of sending this message, the initiator asserts the ATN signal
before it releases ACK for the REQ/ACK handshake of the message that is to be
rejected.
MESSAGE REJECT is issued in response to any messages the drive considers to be
illegal or not supported. When sending to the initiator, the tape device will do so
before requesting any additional message bytes.
NO-OP ( NO OPERATION) (08h)
If a target requests a message, the initiator sends NO-OP if it does not currently have
any other valid message to send. The message is accepted when the drive is acting as a
target, and may be sent when it is an initiator. If NO-OP is received during a
selection, the drive proceeds to the COMMAND phase (provided ATN does not
continue being asserted). The NO-OP message is ignored by the tape drive.
RESTORE POINTERS (03h)
This message is sent from a target to direct the initiator to restore the most recently
saved pointers (for the currently attached logical unit). Pointers to the command, data,
and status locations for the logical unit are restored to the active pointers. Command
and Status pointers are restored to the beginning of the present Command and Status
areas. The Data pointer is restored to the value at the beginning of the data area in the
absence of a SAVE DATA POINTER message, or to the value at the last SAVE
DATA POINTER message that occurred for that logical unit.
When the RESTORE POINTERS message is received as a target, the target switches
to the message in phase and returns MESSAGE REJECT.
SAVE DATA POINTER (02h)
This message, sent from a target to the initiator, saves a copy of the present active
Data pointer for the logical unit currently attached.
As a target, the tape drive sends this message before a disconnect during a data
transfer. It does not send a SAVE DATA POINTER message if it intends to move
directly to STATUS phase. When received as a target, it switches to message in phase
and returns MESSAGE REJECT.
8-12 DLT4000/DLT4500/DLT4700 Cartridge Tape Subsystem