Seagate Ultra160 Product Manual page 105

SCSI Interface Product Manual, Rev. B
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:
a. after a HARD RESET;
b. after a TARGET RESET message;
c. after a power cycle; and
d. after a change in the transceiver mode (e.g., LVD mode to MSE mode).
Any condition that leaves the data transfer agreement in an indeterminate state shall cause the SCSI device to
enter an asynchronous data transfer mode.
A SCSI device may initiate an SDTR message exchange whenever it is appropriate to negotiate a new data
transfer agreement (either synchronous or asynchronous). SCSI devices that are capable of synchronous data
transfers shall not respond to an SDTR message with a MESSAGE REJECT message.
Renegotiation after every selection is not recommended since a significant performance impact is likely.
The SDTR message exchange establishes the permissible transfer periods and the REQ/ACK offsets for all
logical units on the two SCSI devices. This agreement only applies to ST DATA IN phases and ST DATA OUT
phases. COMMAND, MESSAGE, and STATUS phases shall use asynchronous transfers.
The originating SCSI device (the SCSI device that sends the first of the pair of SDTR messages) sets its values
according to the rules above to permit it to receive data successfully. If the responding SCSI device is able to
also receive data successfully with these values (or smaller transfer periods or larger REQ/ACK offsets or
both), it returns the same values in its SDTR message. If it requires a larger transfer period, a smaller REQ/
ACK offset, or both in order to receive data successfully, it substitutes values in its SDTR message as required,
returning unchanged any value not required to be changed. Each SCSI device, when transmitting data, shall
respect the negotiated limits set by the other's SDTR message, but it is permitted to transfer data with larger
transfer periods, smaller synchronous REQ/ACK offsets, or both. The completion of an exchange of SDTR
messages implies an agreement as shown in Table 36.
Table 36: SDTR messages implied agreements
Responding SCSI device Parallel
Protocol Request response
Non-zero REQ/ACK offset
REQ/ACK offset equal to zero
MESSAGE REJECT message
Implied agreement
Synchronous transfer (i.e., each SCSI device transmits data with a
transfer period equal to or greater than and a REQ/ACK offset equal to
or less than the values received in the other device's SDTR message)
with ST DATA IN and ST DATA OUT phases. This is equivalent to proto-
col options of 0h in the PARALLEL PROTOCOL REQUEST message.
Asynchronous transfer.
The originating SCSI device shall set synchronous transfer.
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