User`s guide

Chapter 4 SCSI Interface
SCSI Message System
62 ATL PowerStor L200 Series Library User’s Guide
A Wide Data Transfer Request (WDTR) message exchange is initiated
by a SCSI device whenever a previously arranged transfer width
agreement becomes invalid. The agreement becomes invalid after any
condition, which may leave the data transfer agreement in an
indeterminate state such as:
•
after a hard reset condition
•
after a
BUS DEVICE RESET
message
•
after a power cycle
The
WDTR
message exchange establishes an agreement between two
SCSI devices on the width of the data path to be used for
DATA
phase
transfer between the two devices. This agreement applies to
DATA IN
and
DATA OUT
phases only. All other information transfer phases will
use an eight-bit data path.
The library implements both wide data transfer option and
synchronous data transfer option. It negotiates the wide data transfer
agreement prior to negotiating the synchronous data transfer
agreement. If a synchronous data transfer agreement is in effect, then
after accepting a WDTR message, it resets the synchronous agreement
to asynchronous mode.
The transfer width is two to the transfer width exponent bytes wide.
The transfer width that is established applies to all logical units. Valid
transfer width for the library is 8 bits (
m
= 00h). Values of
m
greater
than (01h) are reserved.