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SCSI MESSAGES M2488 PRODUCT GUIDE
3-6 CG00000-011503 REV. A April 1997
Byte 3: The transfer width is two to the transfer width exponent bytes wide. The transfer width
that is established applies to all logical units on both SCSI devices. Valid transfer widths are 8 bits
(m = 00h), 16 bits (m = 01h), and 32 bits (m = 02h). Values of m greater than 02h are reserved.
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:
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 a) above, the implied agreement for wide data transfers shall be consid-
ered to be negated by both the initiator and the target if the initiator asserts ATN and the first mes-
sage 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 MES-
SAGE PARITY ERROR case, the implied agreement shall be reinstated if a retransmittal 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 changing to any other information
transfer phase and transferring at least one byte of information or by going to the BUS FREE
phase. 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.
If the target recognizes that negotiation is required, it sends a WDTR message to the initiator.
Prior to releasing the ACK signal on the last byte of the WDTR message from the target, the initi-
ator shall assert the ATN signal and respond with its WDTR message or with a MESSAGE
REJECT message. If an abnormal condition 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, indi-
cating 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 mes-
sage, it shall go to the BUS FREE phase without any further information transfer attempt (see
6.1.1). 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.
Responding device WDTR response Implied agreement
a) Non-zero transfer width Each device transmits and receives data with a transfer
width equal to the responding SCSI device's transfer
width.
b) Transfer width equal to zero Eight-bit data transfer
c) MESSAGE REJECT message Eight-bit data transfer