40 Product Manual - Disc Drive SCSI-2/SCSI-3 Interface (Vol. 2; Ver. 2), Rev. E
_____________________________________________________________________________________
If, following an initiator’s responding WDTR message, the target shifts to MESSAGE IN phase and the first
message 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 re-
ceived, until a hard reset condition occurs, or until one of the two SCSI devices elects to modify the agree-
ment. 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
agreement 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 capa-
bility 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 below:
Handshake
8-bit
16-bit
No.
Data Transfers
Data Transfers
1
W
X
W
2
X
Z
Y
3
Y
15 . . . . 8 7. . . . 0
Bit Number
4
Z
7. . . . . . . . . . . . 0
Bit Number
Figure 3.5.3-1. Wide SCSI byte ordering
If the last data byte transferred for a command does not fall on the DB(15-8,P1) signals for a 16-bit wide
transfer, then the values of the remaining higher-numbered bits are undefined. However, parity bits for these
undefined bytes shall be valid for whatever data is placed on the bus.
