Wide Data Transfer Request Message (I↔T) - Fujitsu MAW3073 SERIES Specifications

2.3.21 WIDE DATA TRANSFER REQUEST message (I↔T)
Byte
Two SCSI devices exchange the WIDE DATA TRANSFER REQUEST message to determine the
data bus width between them.
When an SCSI device that supports the wide mode data transfer connects to other SCSI device just
after power-on, after the RESET condition occurs, or after it receives the TARGET RESET
message, it exchanges this message to determine the wide mode data transfer. Each SCSI device
shall always respond to this message exchange request any time when it is issued from another
SCSI device.
The data bus width between two SCSI devices is determined by the exchange of WIDE DATA
TRANSFER REQUEST message. The determined data bus width is applied to the DATA IN and
DATA OUT phases only. In another INFORMATION TRANSFER phase, data is transferred over
the 8-bit bus data.
If an SCSI device supports both the synchronous data transfer and the wide mode transfer, it shall
exchange the WIDE DATA TRANSFER REQUEST message with another SCSI device before
exchanging the SYNCHRONOUS DATA TRANSFER REQUEST message.
If the SCSI device having the wide mode transfer function receives the WIDE DATA TRANSFER
REQUEST message, the data transfer mode is reset to the asynchronous transfer mode.
(1) Wide mode parameters
The data bus width between two SCSI devices is determined by exchanging the WIDE DATA
TRANSFER REQUEST message. This value applies to all logical units assigned to these two
SCSI devices.
The valid data bus width is determined by the Transfer Width Exponent value.
An SCSI device, which sends this message first, specifies the maximum possible data bus width
and sends the WIDE DATA TRANSFER REQUEST message. The destination SCSI device
returns the same value if it can support the requested data bus width. If impossible, the destination
SCSI device answers the possible data bus width.
After the WIDE DATA TRANSFER REQUEST messages have been exchanged normally, the two
SCSI devices shall set the data bus width based on the response (the return message) by the SCSI
device which has first received the message. Table 2.7 defines the data bus width to be set by
message exchange.
2-24
0 1 2
X'01' X'02' X'03'
m: Transfer Width Exponent
m
Transfer width = 2 bytes
3
m
C141-C011