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.
M3099GX/GH OEM Manual |