Tandberg Data
Message System
6-6
Tandberg SLR Product Line SCSI Functional Specifications
6.3.1.
Wide Data Transfer Request
The Wide Data Transfer Request (WDTR) message exchange
establishes an agreement between the Initiator and the
Drive on the width of the data path to be used for DATA
phase transfers between the two devices. The agreement
applies to DATA IN and DATA OUT phases only. All other
information phases will use an eight-bit data path.
A WDTR message exchange is initiated by the Initiator
whenever a previously arranged transfer width agreement
may have become invalid, or whenever it is appropriate
to negotiate a new transfer width agreement. Re-
negotiation at every selection is not recommended, since
a significant performance impact is likely.
The agreement becomes invalid after a reset condition.
The Initiator sets its transfer width to the maximum
data path width it elects to accommodate. If the Drive
can also accommodate this transfer width, it returns the
same value in its WDTR message. If it requires a smaller
transfer width, it substitutes the smaller value in its
WDTR message. The successful completion of an exchange
of WDTR messages implies an agreement as follows:
WDTR Response:
Implied agreement:
1) Non-zero transfer width
Each device transmits and receives data with the specified transfer width.
2) Transfer width equal to zero
Eight-bit data transfer
3) MESSAGE REJECT message
Eight-bit data transfer
If the initiator recognizes that negotiation is
required, it asserts the ATN signal and sends a WDTR
message to begin the negotiating process. After
successfully completing the MESSAGE OUT phase, the Drive
responds with the proper WDTR message. If an abnormal
condition prevents the Drive from returning an
appropriate response, both devices shall go to eight-bit
data transfer mode for data transfers between the two
devices.
Following Drive response (1) above, the implied
agreement for wide data transfers is considered to be
negated if the initiator asserts ATN and the first
message out is either MESSAGE PARITY ERROR or MESSAGE
REJECT. In this case, both devices shall go to eight-bit
data transfer mode for data transfers between the two
devices. For the MESSAGE PARITY ERROR case, the implied
agreement shall be reinstated if a re-transmittal of the
second of the pair of messages is successfully
accomplished. After a specific number of retry attempts,
if the Drive receives a MESSAGE PARITY ERROR message, it
will terminate the retry activity, and both devices
shall go to eight-bit data transfer mode for data
transfers between the two devices. The number of retry
attempts is specified in the Bus Parity Error Retries
field in the Miscellaneous Page of the Mode Select
command. Please refer to section 14.3.9 for further
details.
The implied transfer width agreement shall remain in
effect until a TARGET RESET message is received, until a
hard reset condition occurs, or until the Initiator
