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SCSI Interface Product Manual, Rev. J
Wide Data Transfer Request message
A Wide Data Transfer Request (WDTR) message (Table 9) exchange shall be initiated by a SCSI device when-
ever a previously-arranged transfer width agreement may have become invalid. The agreement becomes
invalid after any condition which 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; and
3. after a power cycle.
Table 9.
Wide Data Transfer Request message
Bit
7
Byte
0
1
2
3
Note.
For SCSI devices that implement WDTR, a WDTR negotiation invalidates a prior SDTR negotiation.
In addition, a SCSI device may initiate a WDTR message exchange whenever it is appropriate to negotiate a
new transfer width agreement. SCSI devices that are capable of wide data transfers (greater than eight bits)
shall not respond to a WDTR message with a Message Reject message.
The WDTR message exchange establishes an agreement between two SCSI devices on the width of the data
path to be used for Data phase transfers between the two devices. This agreement applies to Data In and Data
Out phases only. All other information transfer phases shall use an eight-bit data path.
If a SCSI device implements both wide data transfer option and synchronous data transfer option, then it shall
negotiate the wide data transfer agreement prior to negotiating the synchronous data transfer agreement. If a
synchronous data transfer agreement is in effect, then a SCSI device that accepts a WDTR message shall
reset the synchronous agreement to asynchronous mode.
Byte 3 selects the transfer width in bytes. The number of bytes transfer width is 2
fer Width Exponent given in byte 3. The transfer width that is established applies to all logical units on both
SCSI devices. Valid transfer widths are 8 bits (m = 00h) and 16 bits (m = 01h). Values of m greater than 01h are
not applicable to drives supported by this manual.
The originating SCSI device (the SCSI device that sends the first of the pair of WDTR messages) sets its trans-
fer width value to the maximum data path width it elects to accommodate. If the responding SCSI device 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:
Responding device
WDTR response
(1) Non-zero transfer width
(2) Transfer width equal to zero
(3) Message Reject message
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 target shall respond
with the proper WDTR message. If an abnormal condition prevents the target from returning an appropriate
response, both devices shall go to eight-bit data transfer mode for data transfers between the two devices.
Following target response (1) above, the implied agreement for wide data transfers shall be considered to be
negated by both the initiator and the target if the initiator asserts ATN and the first message out is either Mes-
6
5
Extended Message (01h)
Extended Message Length (02h)
Wide Data Transfer Request Code (03h)
Transfer Width Exponent
Implied agreement
Each device transmits and receives data with a transfer width equal to the
responding SCSI device's transfer width.
Eight-bit data transfer
Eight-bit data transfer
4
3
2
1
m
bytes, where m is the Trans-
37
0
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