FUJITSU M3093DG - Manual (Page 151)

A SYNCHRONOUS DATA TRANSFER REQUEST (SDTR) message exchange
shall be initiated by an 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, an SCSI device may initiate an SDTR message exchange whenever it is
appropriate to negotiate a new data transfer agreement (either synchronous or
asynchronous). SCSI device that are capable of synchronous data transfers shall
not respond to an SDTR message with a MESSAGE REJECT message.
The SDTR message exchange establishes the permissible transfer periods and the
REQ/ACK offsets for all logical units on the two devices. This agreement only
applies to data phases.
Transfer Period Factor: 62, 93, 125, 156 (4.0, 2.6, 2.0, 1.6 [MB/S] )
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.
REQ/ACK Offset: 0 (Async), 1 to 8
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 chosen to prevent overflow conditions in the deviceπs
reception buffer and offset counter. A REQ/ACK offset value of zero shall indicate
asynchronous data transfer mode; a value of FFh shall indicate unlimited
REQ/ACK offset.
The originating device (the device that sends the first of the pair of SDTR message)
sets its values according to the rules above to permit it to receive data successfully.
If the responding device can also receive data successfully with those value (or
smaller transfer periods or larger REQ/ACK offsets or both), it returns the same
values in its SDTR message. If it requires a larger transfer period, a smaller
REQ/ACK offset, or both in order to receive data successfully, it substitutes values
in its SDTR message as required, returning unchanged any value not required to be
changed. Each device when transmitting data shall respect the limits set by the
otherπs SDTR message, but it is permitted to transfer data with larger transfer
periods, smaller REQ/ACK offsets, or both than specified in the otherπs SDTR
message. The successful completion of an exchange of SDTR message implies an
agreement as follows:
4 $ 119