Product Manual - Disc Drive SCSI-2/SCSI-3 Interface (Vol. 2; Ver. 2), Rev. E
39
_____________________________________________________________________________________
Byte 3 selects the transfer width in bytes. The number of bytes transfer width is 2m bytes, where m is the
Transfer 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
transfer 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
Implied Agreement
WDTR Response
(1) Non-zero transfer width
Each device transmits and receives data with a transfer width equal
to the responding SCSI device’s 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 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
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 vendor-specific number of retry attempts (greater than zero), if the target receives a
MESSAGE PARITY ERROR message, it shall terminate the retry activity. This may be done either by chang-
ing to any other information transfer phase and transferring at least one byte of information or by going to the
BUS FREE phase (see 3.1.1). The initiator shall accept such action as aborting the negotiation, and both
devices shall go to eight-bit data transfer mode for data transfers between the two devices.
It the target recognizes that negotiation is required, it sends a WDTR message to the initiator. Prior to releas-
ing the ACK signal on the last byte of the WDTR message from the target, the initiator shall assert the ATN
signal and respond with its WDTR message or with a MESSAGE REJECT message. If an abnormal condi-
tion prevents the initiator from returning an appropriate response, both devices shall go to eight-bit data
transfer mode for data transfers between the two devices.
Following an initiator’s responding WDTR message, an implied agreement for wide data transfer operation
shall not be considered to exist until the target leaves the MESSAGE OUT phase, indicating that the target
has accepted the negotiation. After a vendor-specific number of retry attempts (greater than zero), if the
target has not received the initiator’s responding WDTR message, it shall go to the BUS FREE phase without
any further information transfer attempt (see 3.1.1). This indicates that a catastrophic error condition has
occurred. Both devices shall go to eight-bit data transfer mode for data transfers between the two devices.
