Data Bus Width Defined By Message Exchange - Fujitsu MAN3184 SERIES Specifications

Table 2.7
Response message
WIDE DATA TRANSFER REQUEST
Transfer Width Exponent > 1
WIDE DATA TRANSFER REQUEST
Transfer Width Exponent = 0
MESSAGE REJECT message
(2) Message exchange started by the INIT
If the INIT needs to select the wide data transfer mode, it asserts the ATN signal to start message
exchange. Then, the INIT requests the TARG to receive the WIDE DATA TRANSFER
REQUEST message. When the MESSAGE OUT phase has terminated normally, the TARG shall
return an appropriate WIDE DATA TRANSFER REQUEST message to the INIT. If the TARG
cannot return the message normally, these two SCSI devices shall set the 8-bit data bus width
between them.
If the INIT asserts the ATN signal in the MESSAGE IN phase and sends the MESSAGE REJECT
message even if the TARG returns a WIDE DATA TRANSFER REQUEST message which
indicates a non-zero value in its Transfer Width Exponent field, the two SCSI devices shall set the
8-bit data bus width between them.
If the INIT sends the MESSAGE PARITY ERROR message or the INITIATOR DETECTED
ERROR message against the TARG'S WIDE 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 phase up
to 3 times. If the retry fails, this negotiation is regarded as a failure. The mode setting for wide
transfer between the INIT and the TARG must be set to the 8-bit data bus width.
(3) Message exchange started by the TARG
If the TARG recognizes the need of message exchange and if the wide data mode transfer request
is enabled by the CHANGE DEFINITION command, the TARG sends the WIDE DATA
TRANSFER REQUEST message to the INIT. The INIT shall assert the ATN signal and return the
WIDE DATA TRANSFER REQUEST message or the MESSAGE REJECT message before
negating the ACK signal of the last byte of WIDE DATA TRANSFER REQUEST message which
has been sent from the TARG. If the INIT cannot respond to this message, the two SCSI devices
shall set the 8-bit data bus width between them.
Both the INIT and the TARG shall not consider the completion of WIDE DATA TRANSFER
REQUEST message exchange until the TARG terminates the MESSAGE OUT phase by entering
another INFORMATION TRANSFER phase (which indicates that no parity error has been
detected) even if the INIT responds with the WIDE DATA TRANSFER REQUEST message. If
the TARG cannot normally receive a response message by the INIT after the specified number of
retries, the TARG shall immediately negate the BSY signal and enter the BUS FREE phase
without starting another INFORMATION TRANSFER phase.
The INIT shall consider it as the unsuccessful message exchange, and the two SCSI devices shall
set the 8-bit data bus width between them.

Data bus width defined by message exchange

Data is transferred using the responded data bus width.
8-bit data transfer
8-bit data transfer
C141-E123-01EN
Data bus width
2 - 25