Seagate Ultra 160 Manual De Usuario
Parallel SCSI Interface Product Manual, Rev. A
97
4.3.16
SYNCHRONOUS DATA TRANSFER REQUEST
Synchronous Data Transfer Request (SDTR) messages (see Table 36) are used to negotiate a synchronous
data transfer agreement between two SCSI devices.
data transfer agreement between two SCSI devices.
Table 36:
Synchronous Data Transfer Request message format
The TRANSFER PERIOD FACTOR field values are defined below.
The REQ/ACK OFFSET field contains the maximum number of REQ assertions allowed to be outstanding
before a corresponding ACK assertion is received at the target. The size of a data transfer may be 1 or 2 bytes
depending on what values, if any, have been previously negotiated through an exchange of WIDE DATA
TRANSFER REQUEST messages or PPR messages. The REQ/ACK OFFSET value is chosen to prevent
overflow conditions in the SCSI device's reception buffer and offset counter. A REQ/ACK OFFSET value of
zero shall indicate asynchronous data transfer mode and that the TRANSFER PERIOD FACTOR field shall be
ignored; a value of FFh shall indicate unlimited REQ/ACK OFFSET.
before a corresponding ACK assertion is received at the target. The size of a data transfer may be 1 or 2 bytes
depending on what values, if any, have been previously negotiated through an exchange of WIDE DATA
TRANSFER REQUEST messages or PPR messages. The REQ/ACK OFFSET value is chosen to prevent
overflow conditions in the SCSI device's reception buffer and offset counter. A REQ/ACK OFFSET value of
zero shall indicate asynchronous data transfer mode and that the TRANSFER PERIOD FACTOR field shall be
ignored; a value of FFh shall indicate unlimited REQ/ACK OFFSET.
An SDTR agreement applies to all logical units of the two SCSI devices that negotiated agreement. That is, if
SCSI device A, acting as an initiator, negotiates a synchronous data transfer agreement with SCSI device B (a
target), then the same data transfer agreement applies to SCSI devices A and B even if SCSI device B
changes to an initiator.
SCSI device A, acting as an initiator, negotiates a synchronous data transfer agreement with SCSI device B (a
target), then the same data transfer agreement applies to SCSI devices A and B even if SCSI device B
changes to an initiator.
An SDTR agreement only applies to the two SCSI devices that negotiate the agreement. Separate synchro-
nous data transfer agreements are negotiated for each pair of SCSI devices. The synchronous data transfer
agreement only applies to DATA phases.
nous data transfer agreements are negotiated for each pair of SCSI devices. The synchronous data transfer
agreement only applies to DATA phases.
An SDTR message exchange shall be initiated by a SCSI device whenever a previously arranged synchronous
data transfer 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:
data transfer 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:
Bit
Byte
7
6
5
4
3
2
1
0
0
EXTENDED MESSAGE (01h)
1
EXTENDED MESSAGE LENGTH (03h)
2
SYNCHRONOUS DATA TRANSFER REQUEST (01h)
3
TRANSFER PERIOD FACTOR [1]
4
REQ/ACK OFFSET
Code
Description
00h-07h
Reserved. Faster timings may be allowed by future SCSI parallel interface standards
08h
Transfer period equals 6.25 ns. Fast-160. DT transfers are required for this transfer period factor.
09h
Transfer period equals 12.5 ns. Fast-80. DT transfers are required for this transfer period factor.
0Ah
Transfer period equals 25 ns. Fast-40 data is latched every 25 ns or 30.3 ns.
0Bh
Transfer period equals 30.3 ns. Fast-40 data is latched every 25 ns or 30.3 ns.
0Ch
Transfer period equals 50 ns. Fast-20 data is latched using a transfer period of less than or equal to 96 ns
and greater than or equal to 50 ns.
and greater than or equal to 50 ns.
0D-18h
Transfer period equals the period factor x 4. Fast-20 data is latched using a transfer period of less than or
equal to 96 ns and greater than or equal to 50 ns.
equal to 96 ns and greater than or equal to 50 ns.
19h-31h
Transfer period equals the period factor x 4. Fast-10 data is latched using a transfer period of less than or
equal to 196 ns and greater than or equal to 100 ns.
equal to 196 ns and greater than or equal to 100 ns.
32h-FFh
Transfer period equals the period factor x 4. Fast-5 data is latched using a transfer period of less than or
equal to 1,020 ns and greater than or equal to 200 ns.
equal to 1,020 ns and greater than or equal to 200 ns.