Figure 7. Wide Scsi Byte Ordering - Seagate SCSI Interface Product Manual

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sage 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 rein-
stated 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 changing to any other information transfer phase
and transferring at least one byte of information or by going to the Bus Free phase (see Section 3.1.1). The ini-
tiator 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 releasing
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 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, 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 Section 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.
If, following an initiator's responding WDTR message, the target shifts to Message In phase and the first mes-
sage in is Message Reject, the implied agreement shall be considered to be negated and both devices shall go
to eight-bit data transfer mode for data transfers between the two devices.
The implied transfer width agreement shall remain in effect until a Bus Device Reset message is received, until
a hard reset condition occurs, or until one of the two SCSI devices elects to modify the agreement. The default
data transfer width is eight-bit data transfer mode. The default data transfer mode is entered at power on, after
a Bus Device Reset message, or after a hard reset condition.
16-bit Wide Data Transfer
Wide data transfer is optional and may be used in the Data phase only if a nonzero wide data transfer agree-
ment is in effect. The messages determine the use of wide mode by both SCSI devices and establish a data
path width to be used during the Data phase. Default for the drives that have wide data transfer capability is
8-bit data transfers.
During 16-bit wide data transfers, the first and second logical data bytes for each data phase shall be trans-
ferred across the DB(7-0,P) and DB(15-8,P1) signals respectively, on the 68 conductor cable used by drives
that support the wide data transfer feature. Subsequent pairs of data bytes are likewise transferred in parallel
across the 68 conductor cable.
To illustrate the order of transferring data bytes across the interface assume an example four byte data transfer
of bytes W, X, Y, and Z.
When transferring bytes W, X, Y, and Z across the 8- or 16-bit bus widths, they are transferred in the order
shown in Figure 7.
Handshake
Figure 7. Wide SCSI byte ordering
8-bit
No. Data Transfers
1 W
2
3
4
7. . . . . . . . . . . . 0
Bit Number
SCSI Interface Product Manual, Rev. J
Data Transfers
X
X Z
15 . . . . 8 7. . . . 0
Y
Bit Number
Z
16-bit
W
Y