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5.SCSI Interface
5-8 SONY AIT-5 drive SDX-1100V series Ver.1.0
The implied synchronous agreement shall remain in effect until a BUS DEVICE RESET message is received, until
a hard reset condition occurs, or until one of the two SCSI devices elects to modify the agreement. The default
data transfer mode is asynchronous data transfer mode. The default data transfer mode is entered at power on,
after a BUS DEVICE RESET message, or after a hard reset condition.
5.3.2.3. WIDE DATA TRANSFER REQUEST
Byte Value
0 01 Extended message
1 02 Extended message length
2 03 WIDE DATA TRANSFER REQUEST code
3 m Transfer Width exponent
A WIDE DATA TRANSFER REQUEST (WDTR) message exchange shall be initiated by a SCSI device whenever
a previously-arranged transfer width agreement may have become invalid. The agreement becomes invalid after
any condition which may leave the data transfer agreement in an indeterminate state such as:
(1) after a hard reset condition;
(2) after a BUS DEVICE RESET message and;
(3) after a power cycle.
In addition, a SCSI device may initiate a WDTR message exchange whenever it is appropriate to negotiate a new
transfer width agreement. SCSI devices that are capable of wide data transfers (greater than eight bits) shall not
respond to a WDTR with a MESSAGE REJECT message.
The WDTR message exchange establishes an agreement between two SCSI devices on the width of the data
path to be used for DATA phase transfers between the two devices. This agreement applies to DATA IN and DATA
OUT phases only. All other information transfer phases shall use an eight-bit data path.
If a SCSI device implements both wide data transfer option and synchronous data transfer option, then it shall
negotiate the wide data transfer agreement prior to negotiating the synchronous data transfer agreement. If a
synchronous data transfer agreement is in effect, then a SCSI device that accepts a WDTR message shall reset
the synchronous agreement to asynchronous mode.
The transfer width is two to the transfer width exponent bytes wide. The transfer width that is established applies
to logical units on both SCSI devices. Valid transfer widths are 8 bits (m=00h), and 16 bits (m=01h). Values of
m=02 and greater are reserved.
The originating SCSI device (the SCSI device that sends the first of the pair of WDTR messages) sets its transfer
width value to maximum data path width it elects to accommodate. If the responding SCSI device can also
accommodate this transfer width, it returns the same value in its WDTR message. If it requires a smaller transfer
width, it substitutes the smaller value in its WDTR message. The successful completion of an exchange of WDTR
messages implies an agreement as follows:
Responding device WDTR response Implied agreement
a) Non-ZERO transfer width Each device transmits and receives data with a transfer
width equal to the responding SCSI device's transfer width.
b) Transfer width equal to ZERO Eight -bit data transfer
c) MESSAGE REJECT message Eight -bit data transfer
If the initiator recognizes that negotiation is required, it asserts the ATN signal and sends a WDTR message to
begin the negotiating process. After successfully completing the MESSAGE OUT phase, the target shall respond
with the proper WDTR message. If an abnormal condition prevents the target from returning an appropriate
response, both devices shall go to eight-bit data transfer mode for data transfers between the two devices.