Fujitsu MAW3073 SERIES Specifications page 175

When a SCSI device executes data transfer, it must not send REQ or ACK pulses exceeding the
parameter limits which have been set by the two SCSI devices by exchange of the
SYNCHRONOUS DATA TRANSFER REQUEST message. However, the SCSI device can
transfer data using a larger Transfer Period or a smaller REQ/ACK Offset.
After normal exchange of the SYNCHRONOUS DATA TRANSFER REQUEST message, the two
SCSI devices shall set the data transfer mode depending on the response (the returned message) by
the SCSI device which has first received this message. Table 2.3 defines the data transfer mode to
be selected.
Table 2.3
Responded message
SYNCHRONOUS DATA TRANSFER
REQUEST
REQ/ACK Offset ≥ 1
SYNCHRONOUS DATA TRANSFER
REQUEST
REQ/ACK Offset =0
MESSAGE REJECT
(2) Procedures of the message exchange initiated by the INIT
If the INIT has recognized the synchronous data transfer to be set, it asserts the ATN signal that is,
generating an ATTENTION condition for message exchange, and requests the TARG to receive
the SYNCHRONOUS DATA TRANSFER REQUEST message.
After the MESSAGE OUT phase has completed normally, the TARG shall return the
SYNCHRONOUS DATA TRANSFER REQUEST message or the MESSAGE REJECT message
to the INIT. If the ACK signal is still true after the TARG has received the SYNCHRONOUS
DATA TRANSFER REQUEST message, the TARG can cancel the MESSAGE OUT phase and
can enter the MESSAGE IN phase to return the message. If the TARG cannot return the message,
the two SCSI devices shall select the asynchronous data transfer mode between them.
When the INIT sends the MESSAGE REJECT message as the first byte of the MESSAGE OUT
phase which follows the MESSAGE IN phase, this negotiation is regarded as a failure and the
mode setting for synchronous transfer between the INIT and the TARG must be set to the
asynchronous mode. This event occurs even if the TARG returns the SYNCHRONOUS DATA
TRANSFER REQUEST message with specifying 1 or more to the REQ/ACK Offset value.
If the INIT sends the MESSAGE PARITY ERROR message or the INITIATOR DETECTED
ERROR message against the TARG's SYNCHRONOUS DATA TRANSFER REQUEST message,
the IDD recognizes that there was a parity error in the previous MESSAGE IN phase. Then, the
IDD performs the error recovery procedure based on the SCSI Bus protocol for the erroneous
C141-C011
Definition of data transfer mode by message exchange
Synchronous mode:
Each SCSI device executes data transfer using the
Transfer Period value equal to or greater than the
specified value and REQ/ACK Offset value equal
to or smaller than the specified value specified in
the SYNCHRONOUS DATA TRANSFER
REQUEST message from the opposite SCSI
device.
Data transfer mode
Asynchronous mode
Asynchronous mode
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