Fujitsu MAW3073 SERIES Specifications page 190

A PARALLEL PROTOCOL REQUEST message exchange shall be initiated by a SCSI device
whenever a previously 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) RESET condition ("hardware RESET") occurrence
b) TARGET RESET message reception
c) Power turned off
d) Change in the transceiver mode (e.g., LVD mode to SE mode)
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.
A SCSI device 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 MESSAGE 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 agreement 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 originating SCSI device (the SCSI device that sends
the first of the pair of PARALLEL PROTOCOL REQUEST messages) sets its values
according to the rules above to permit it to receive data successfully.
If the responding SCSI device 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 2.12
If the responding SCSI device does not support the selected protocol option it shall clear as many
bits as required to set the protocol option field to a legal value that it does support.
If there is an unrecoverable parity error on the initial PARALLEL PROTOCOL REQUEST
message the initiating SCSI device shall retain its previous data transfer mode and protocol
options. If there is an unexpected bus free on the initial PARALLEL PROTOCOL REQUEST
message the initiating SCSI device shall retain its previous data transfer mode and protocol
options.
2-32 C141-C011