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34
Target Transfer Disable (13h)
The Target Transfer Disable (TDD) message is sent from an initiator to a target to request that subsequent
reconnections for data transfer on the task be done by the initiator instead of the target. The target may recon-
nect for other purposes, but shall not enter a data phase on a target reconnection. SCSI devices that imple-
ment this message shall also implement the Continue Task message.
If used, this message shall be sent as the last message of the first Message Out phase of an initial connection.
The target may continue the task, including any Data Out phases on the initial connection, until the target
would normally disconnect, but the target shall not reconnect to transfer data. That is, the target shall not enter
a Data In phase on the initial connection and the target shall not enter any data phase on any subsequent tar-
get reconnection for the task.
When the target is ready to transfer data for a disconnected task for which a TTD message has been sent, the
target shall reconnect to the initiator for the task (via a Reselection phase, an Identify message, and an
optional Simple Task message), send a Disconnect message, and, if the initiator does not respond with a Mes-
sage Reject message, go to the Bus Free phase. This connection serves to notify the initiator that the task is
ready for data transfer. If the initiator rejects the Disconnect message, the target may enter a data phase; oth-
erwise, the initiator may reconnect to the task as described in the Continue Task message (see page 30) to do
the data transfer.
Initiators should avoid sending the TTD message to targets which have not implemented this message. Such
targets may not respond as described in this section. An initiator can determine whether a target implements
this message by examining the TrnDis bit in the standard Inquiry data (see Paragraph 5.6).
3.5.3.2
Extended Message (01h)
Extended messages are messages that require more than two bytes to send the necessary information. The
Extended messages supported by the drive are Modify Data Pointer and Synchronous Data Transfer. These
two are described in detail in the following paragraphs.
Modify Data Pointer message
The Modify Data Pointer message (Table 6) is sent from the target to the initiator and requests that the signed
argument be added (two's complement) to the value of the current data pointer.
Table 6:
Modify Data Pointer message
Byte
Value
0
01h
1
05h
2
00h
3
4
5
6
Synchronous Data Transfer Request message
A synchronous data transfer agreement only applies to the two SCSI devices that negotiate the agreement.
Separate synchronous data transfer agreements are negotiated for each pair of SCSI devices. The synchro-
nous data transfer agreement only applies to data phases.
A Synchronous Data Transfer Request (SDTR) message (Table 7) exchange shall be initiated by a SCSI
device whenever a previously arranged data transfer agreement may have become invalid. The agreement
becomes invalid after any condition that may leave the data transfer agreement in an indeterminate state such
as:
1. after a hard reset condition,
2. after a Bus Device Reset message,
3. after a power cycle, and
4. for SCSI devices that implement WDTR, occurrence of an intervening WDTR.
Description
Extended message
Extended message length
Modify Data Pointer code
Argument (Most Significant Byte)
Argument
Argument
Argument (Least Significant Byte)
SCSI Interface Product Manual, Rev. J
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