Fujitsu MAN3184 SERIES Specifications page 148

A PARALLEL PROTOCOL REQUEST agreement applies to all logical units of the two SCSI
devices that negotiated agreement. That is, if SCSI device A, acting as an INIT negotiates a data
transfer agreement with SCSI device B (a TARG), then the same data transfer agreement applies to
SCSI devices A and B even if SCSI device B changes to an INIT.
A data transfer agreement only applies to the two SCSI devices that negotiate the agreement.
Separate data transfer agreements are negotiated for each pair of SCSI devices. The data transfer
agreement only applies to DATA phases and information unit transfers.
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.
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