Seagate ST3285N, Руководство по эксплуатации продукта

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A.2 Synchronous data transfer request message (01
H
)
The synchronous data transfer message is the only extended message
supported by the drive.
Depending on the value contained in the SSM bit (contained in byte 2 of
the Operating page in Section C.10), the drive or the initiator can
negotiate for synchronous data transfer after a reset. If any problem
precludes the successful exchange of synchronous data transfer request
messages, the initiator and drive default to asynchronous data transfers.
This exchange of messages establishes the transfer period and the
REQ/ACK offset.
Bit
Byte
7 6 5 4 3 2 1 0
0 Extended message (01
H
)
1 Extended message length (03
H
)
2 Identifier code (01
H
)
3 Minimum transfer period divided by 4
4 REQ/ACK offset
Byte 0 This byte identifies the message as an extended message.
Byte 1 This byte reports the length of the message.
Byte 2 This byte identifies the message as a synchronous data transfer
request message.
Byte 3 The value contained in this byte is in nanoseconds. It is equal to
the minimum time between leading edges of successive REQ
and ACK pulses divided by four. In byte 3, the minimum value
supported by the drive is 25, which is equivalent to a transfer
period of 100 nanoseconds, or an external transfer rate of
10 Mbytes per second. A value of 50 is equivalent to a transfer
period of 200 nanoseconds, or an external transfer rate of
5 Mbytes per second.
Byte 4 The REQ/ACK offset is the maximum number of REQ pulses
that may be outstanding before its corresponding ACK pulse is
received at the target. A REQ/ACK offset of zero indicates
asynchronous mode. The drive supports a maximum REQ/ACK
offset of six.
78 ST3655 Family SCSI Drives Product Manual, Rev. A