Fujitsu manual M2488 Product Guide Scsi Messages, Extended Message Format code 01h

M2488 PRODUCT GUIDE SCSI MESSAGES

3-2.5 EXTENDED MESSAGE FORMAT code 01h

This message is sent from either the initiator or the target as the first byte of a multiple-byte message.

		CODE						MESSAGE			LENGTH
											(BYTES)
			01h		Synchronous Data Transfer Request						5
			03h		Wide Data Transfer Request						4
3-2.5.1	Synchronous Data Transfer Request (SDTR)
								BITS
BYTES	7		6			5		4	3	2		1	0
0								Extended Message (01h)
1							Extended Message Length (03h)
2				SYNCHRONOUS DATA TRANSFER REQUEST code (01h)
3								Transfer Period Factor
4								REQ/ACK Offset

A SYNCHRONOUS DATA TRANSFER REQUEST (SDTR) message exchange is initiated by a SCSI device whenever a previously arranged data transfer 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:

a)after a hard reset condition;

b)after a BUS DEVICE RESET message and;

c)after a power cycle.

In addition, a SCSI device may initiate an SDTR message exchange whenever it is appropriate to negotiate a new data transfer agreement (either synchronous or asynchronous). SCSI devices that are capable of synchronous data transfers shall not respond to an SDTR message with a MES- SAGE REJECT message.

Renegotiation at every selection is not recommended, since a significant performance impact is likely.

The SDTR message exchange establishes the permissible transfer periods and the REQ/ACK off- sets for all logical units and target routines on the two devices. This agreement only applies to data phases.

Byte 3: The transfer period factor times four is the value of the transfer period. The transfer period is the minimum time allowed between leading edges of successive REQ pulses and of successive ACK pulses to meet the device requirements for successful reception of data.

Byte 4: The REQ/ACK offset is the maximum number of REQ pulses allowed to be outstanding before the leading edge of its corresponding ACK pulse is received at the target. This value is cho- sen to prevent overflow conditions in the device's reception buffer and offset counter. A REQ/ ACK offset value of zero shall indicate asynchronous data transfer mode; a value of FFh shall indi- cate unlimited REQ/ACK offset.

April 1997 CG00000-011503 REV. A 3-3