Seagate Ultra 320 ユーザーズマニュアル

Parallel SCSI Interface Product Manual, Rev. A                                        

   97

Synchronous Data Transfer Request (SDTR) messages (see Table 36) are used to negotiate a synchronous 
data transfer agreement between two SCSI devices.

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: 

EXTENDED MESSAGE (01h)

EXTENDED MESSAGE LENGTH (03h)

SYNCHRONOUS DATA TRANSFER REQUEST (01h)

TRANSFER PERIOD FACTOR [1]

REQ/ACK OFFSET

Reserved. Faster timings may be allowed by future SCSI parallel interface standards

Transfer period equals 6.25 ns. Fast-160. DT transfers are required for this transfer period factor.

Transfer period equals 12.5 ns. Fast-80. DT transfers are required for this transfer period factor.

Transfer period equals 25 ns. Fast-40 data is latched every 25 ns or 30.3 ns.

Transfer period equals 30.3 ns. Fast-40 data is latched every 25 ns or 30.3 ns.

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. 

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.

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.

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.