Seagate Ultra 160, Ultra 320 manual Synchronous Data Transfer Request

4.3.16SYNCHRONOUS 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

Bit	7	6	5		4	3	2	1	0
Byte
0				EXTENDED MESSAGE (01h)
1				EXTENDED MESSAGE LENGTH (03h)
2			SYNCHRONOUS DATA TRANSFER REQUEST (01h)
3				TRANSFER PERIOD FACTOR [1]
4					REQ/ACK OFFSET

The TRANSFER PERIOD FACTOR field values are defined below.

Code 00h-07h

08h

09h

0Ah

0Bh

0Ch

0D-18h

19h-31h

32h-FFh

Description

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.

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:

Parallel SCSI Interface Product Manual, Rev. A 97