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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.
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.
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.
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.
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:
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.
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.
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.
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.