Fujitsu M2488 manual Wide Data Transfer Request Wdtr, Bits Bytes

M2488 PRODUCT GUIDE SCSI MESSAGES

(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)

BITS

BYTES	7	6	5		4	3	2	1	0
0					Extended Message (01h)
1				Extended Message Length (02h)
2			WIDE DATA TRANSFER REQUEST code (03h)
3					Transfer Width Exponent

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 CG00000-011503 REV. A 3-5