Seagate Ultra160 Product Manual page 102

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A PARALLEL PROTOCOL REQUEST message exchange shall be initiated by an initiator whenever a previ-
ously arranged parallel protocol agreement may have become invalid. The agreement becomes invalid after
any condition that may leave the parallel protocol agreement in an indeterminate state such as:
a. after a hard reset;
b. after a TARGET RESET message;
c. after a power cycle; or
d. after a change in the transceiver mode (e.g., LVD mode to SE mode).
If a target determines that the agreement is in an indeterminate state, it shall initiate a negotiation to enter an
asynchronous eight-bit wide data transfer mode with all the protocol options bits set to zero, using a WDTR
message with the transfer bit exponent set to 00h.
Any condition that leaves the data transfer agreement in an indeterminate state shall cause the SCSI device to
enter an asynchronous, eight-bit wide data transfer mode with all the protocol options bits set to set to zero.
An initiator may initiate a PARALLEL PROTOCOL REQUEST message exchange whenever it is appropriate to
negotiate a data transfer agreement. SCSI devices that are currently capable of supporting any of the Parallel
Protocol Request options shall not respond to a PARALLEL PROTOCOL REQUEST message with a MES-
SAGE REJECT message.
Renegotiation after every selection is not recommended, since a significant performance impact is likely.
The Parallel Protocol Request message exchange establishes an agreement between the two SCSI devices;
a. on the permissible periods and the REQ/ACK offsets for all logical units on the two SCSI devices. This
agreement only applies to ST DATA IN phases, ST DATA OUT phases, DT DATA IN phases, and DT DATA
OUT phases. All other phases shall use asynchronous transfers;
b. on the width of the data path to be used for Data phase transfers between two SCSI devices. This agree-
ment only applies to ST DATA IN phases, ST DATA OUT phases, DT DATA IN phases, and DT DATA OUT
phases. All other information transfer phases shall use an eight-bit data path; and
c. on the protocol option is to be used.
The initiator sets its values according to the rules above to permit it to receive data successfully. If the target is
able to receive data successfully with these values (or smaller periods or larger REQ/ACK offsets or both), it
returns the same values in its PARALLEL PROTOCOL REQUEST message. If it requires a larger period, a
smaller REQ/ACK offset, or a smaller transfer width in order to receive data successfully, it substitutes values in
its PARALLEL PROTOCOL REQUEST 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
PARALLEL PROTOCOL REQUEST message, but it is permitted to transfer data with larger periods, smaller
synchronous REQ/ACK offsets, or both. The completion of an exchange of PARALLEL PROTOCOL
REQUEST messages implies an agreement as shown in Table 34.
If the target does not support the selected protocol option it shall clear as many bits as required to set the pro-
tocol option field to a legal value that it does support.
SCSI Interface Product Manual, Rev. B