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3-108 M3099GX/GH OEM Manual
A SYNCHRONOUS DATA TRANSFER REQUEST (SDTR)
message (Table 3.87) 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 devices 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(4MB/s max.) to 255
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 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
indicate unlimited REQ/ACK offset.
#03.pm5 98.2.22, 1:34 PM108