3-2.5.2 Wide Data Transfer Request (Wdtr) - Fujitsu M2488 Product Manual

M2488 PRODUCT GUIDE
(greater than zero), if the target has not received the initiator's responding SDTR message, it shall
go to the BUS FREE phase without any further information transfer attempt. This indicates that a
catastrophic error condition has occurred. Both devices shall go to asynchronous data transfer
mode for data transfers between the two devices.
If, following an initiator's responding SDTR message, the target shifts to MESSAGE IN phase and
the first message in is MESSAGE REJECT, the implied agreement shall be considered to be
negated and both devices shall go to asynchronous data transfer mode for data transfers between
the two devices.
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 mod-
ify 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.
3-2.5.2 Wide Data Transfer Request (WDTR)
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A WIDE DATA TRANSFER REQUEST (WDTR) message exchange is initiated by a SCSI device
whenever a previously arranged transfer width agreement may have become invalid. The agree-
ment becomes invalid after any condition which may leave the data transfer agreement in an inde-
terminate state such as:
a) after a hard reset condition;
b) after a BUS DEVICE RESET message and;
c) after a power cycle.
In addition, an 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) do not respond to a WDTR message with a MESSAGE REJECT message.
Renegotiation at every selection is not recommended, since a significant performance impact is
likely.
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 an 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 an SCSI device that
accepts a WDTR message shall reset the synchronous agreement to asynchronous mode.
April 1997
6 5 4
Extended Message (01h)
Extended Message Length (02h)
WIDE DATA TRANSFER REQUEST code (03h)
Transfer Width Exponent
CG00000-011503 REV. A
BITS
3 2
SCSI MESSAGES
1 0
3-5