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100 Parallel SCSI Interface Product Manual, Rev. A
4.3.18 WIDE DATA TRANSFER REQUEST
WIDE DATA TRANSFER REQUEST (WDTR) messages (see Table 38) are used to negotiate a wide data
transfer agreement between two SCSI devices.
Table 38: WIDE DATA TRANSFER REQUEST message format
The TRANSFER WIDTH EXPONENT field defines the transfer width to be used during ST DATA IN phases
and ST Data Out phases. The transfer width that is established applies to both SCSI devices. Valid transfer
widths are 8 bits (m=00h) and 16 bits (m=01h). A Transfer Width Exponent Field value of 02h is obsolete and
values greater than 02h are reserved.
A WDTR agreement applies to all logical units of the two SCSI devices that negotiated agreement. That is, if
SCSI device A, acting as an initiator, negotiates a wide data transfer agreement with SCSI device B (a target),
then the same transfer width agreement applies to SCSI devices A and B even if SCSI device B changes to an
initiator.
A WDTR only applies to the two SCSI devices that negotiate the agreement. Separate wide transfer agree-
ments are negotiated for each pair of SCSI devices. The wide data transfer agreement only applies to Data
phases.
A WDTR message exchange shall be initiated by a SCSI device whenever a previously arranged wide transfer
agreement may have become invalid. The agreement becomes invalid after any condition that may leave the
wide transfer agreement in an indeterminate state such as:
a. after a hard reset;
b. after a TARGET RESET message;
c. after a power cycle; and
d. after a change in the transceiver mode (e.g., LVD mode to MSE mode).
Any condition that leaves the data transfer agreement in an indeterminate state shall cause the SCSI device to
enter an eight-bit wide data transfer mode.
A SCSI device may initiate a WDTR message exchange whenever it is appropriate to negotiate a new wide
transfer agreement. SCSI devices that are capable of wide data transfers (greater than 8 bits) shall not
respond to a WDTR message with a MESSAGE REJECT message.
Renegotiation after every selection is not recommended, since a significant performance impact is likely.
The WDTR message exchange establishes an agreement between the two SCSI devices on the width of the
data path to be used for Data phase transfers between two SCSI devices. This agreement only applies to ST
DATA IN phases and ST DATA OUT phases. All other information transfer phases, except DT Data phases,
shall use an eight-bit data path.
If a SCSI device implements both wide data transfer option and synchronous data transfer option and uses the
SDTR and WDTR messages, 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. If a WDTR message is rejected with a MESSAGE REJECT message, the prior synchronous data trans-
Bit
Byte
7 6 5 4 3 2 1 0
0 EXTENDED MESSAGE (01h)
1 EXTENDED MESSAGE LENGTH (02h)
2 WIDE DATA TRANSFER REQUEST (03h)
3 TRANSFER WIDTH EXPONENT