Reference Guide

Chapter 3 Transport Layer Implementation Details
Parallel SCSI
DLT-S4 Interface Reference Guide 17
Wide Data Transfer Request 3Figure 4 illustrates the message formats.
Figure 4 Wide Data Transfer
Request Message — Data
Format
The DLT-S4 tape drive initiates a WIDE DATA TRANSFER REQUEST
message exchange whenever a previously arranged transfer width
agreement may have become invalid. The agreement becomes invalid
after any condition that may leave the data transfer agreement in an
indeterminate state, such as:
• After a hard reset condition
•After a
TARGET RESET message
• After a power cycle.
The
WIDE DATA TRANSFER REQUEST message exchange establishes an
agreement between two SCSI devices on the width of the data path to be
used for
DATA phase transfer between them. This agreement applies to
DATA IN and DATA OUT phases only. All other information transfer phases
must use an eight-bit data path.
The drive implements both the wide data transfer option and
synchronous data transfer option. If both wide and synchronous data
transfers are used, the wide data transfer agreement must be negotiated
first. If a synchronous data transfer agreement is in effect, the drive resets
the synchronous agreement to asynchronous mode after accepting a
WIDE
DATA TRANSFER REQUEST
message.
The transfer width established applies to all logical units. Valid transfer
widths for the tape drive are 8 bits (transfer width = 00h) and 16 bits
(transfer width = 01h). Other transfer widths are reserved.
Bit
7 6 5 4 3 2 1 0
Byte
0 Extended Message Identifier (01h)
1 Length (02h)
2 WIDE DATA TRANSFER REQUEST (03h)
3 Transfer Width Exponent