Chapter 3 Transport Layer Implementation Details
Parallel SCSI
DLT-S4 Interface Reference Guide
14
• After any SCSI reset condition
• After a
TARGET RESET
message
• After a power cycle
• After a
WIDE DATA TRANSFER REQUEST
message exchange.
The
SYNCHRONOUS DATA TRANSFER REQUEST
message exchange
establishes an agreement between two SCSI devices on the clocking of the
data used for
DATA
phase transfer between them. This agreement applies
to the
DATA IN
and
DATA OUT
phases only. All other information transfer
phases must use asynchronous transfers.
The tape drive implements both the wide data transfer and synchronous
data transfer options. 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.
If the Transfer Period requested is less than the minimum value the drive
supports, the drive adjusts the return value up to the minimum
supported value.
Not all possible transfer periods between the minimum and maximum
values are supported. If the Transfer Period requested is between the
minimum and maximum supported values, but not exactly achievable by
the drive, the drive returns the request value and the drive transmits data
at the next lower speed it is capable of. The initiator may send data at the
request speed. The maximum supported synchronous period is 5Dh
(372 nsec). A request with a Transfer Period less than this returns a
request for asynchronous mode (see
table 4
).
Table 4 Transfer Periods and
Transfer Rates
Transfer Period
Transfer Rate
0Ah
40 MHz, 25 nsec Transfer Period
0Bh
33 MHz, 30.3 nsec Transfer Period; Ultra 2 Super
DLTtape uses 37.5 nsec period
0Ch
20 MHz, 50 nsec Transfer Period
0Dh – 5Dh
(4 * Transfer Period) nsec
