Section 5
SCSI Programming Guidelines
SYNCHRONOUS
DATA
TRANSFER
REQUEST:
Byte
Value
Description
0
01h
Extended Message
1
03h
Extended
Mes~e
Len~th
2
01h SYNCHRONOUS DATA TRANSFER REQUEST code
3 m
Transfer
Period (m times 4 nanoseconds)
4 x REQ/ACK
Offset
The transfer period is the clock rate for the transfer. The minimum transfer period for the ProDrive
is 250 ns
or
62 (3EH). This value is equivalent to a maximum transfer rate
of
4.0 MBytes/second.
Table 5-3 shows the transfer rates supported and the transfer period values for the
ProDrive.
The REQ/ ACK offset is the maximum number
of
REQ
pulses that may be sent before a
corresponding
ACK
pulse is received at the ProDrive. A REQ/ACK offset value
of
zero indicates
asynchronous mode. For the
ProDrive, the maximum value
of
REQ/ACK offset is 8.
If
the initiator recognizes that negotiation is required, it asserts ATN, sends a SYNCHRONOUS
DATA TRANSFER REQUEST message indicating a REQ/ACK offset and minimum transfer
period. The REQ/ ACK offset is chosen to prevent initiator buffer overflows, while the minimum
transfer period is chosen to meet the initiator's data handling requirements. The
ProDrive responds
in one
of
the following ways:
ProDrive
Response
1)
REQ/ACK offset less than or
equal to the requested value;
minimum transfer period equal
to
or greater than requested
period.
2)
REQ/ACK offset equal
to
zero.
3)
MESSAGE REJECT.
Implied Agreement
REQ/ ACK offset equal to ProDrive
value; minimum transfer period equal
to the
ProDrive value.
Asynchronous transfer.
Asynchronous transfer.
TABLE
5-3 Transfer Rates Supported/Transfer Period Values
TRANSFER PERIOD VALUE
TRANSFER RATE IN MHZ
DEC HEX
0-62
00
- 3EH
4.00
63 - 93
3FH - 5DH
2.67
94 - 125
5EH
-7DH
2.00
126 - 156
7EH - 9CH
1.60
157 - 187
9~-BBH
1.33
188
- 218
BCH
-DAH
1.14
219 -
250
DBH
-FAH
1.00
251
- 255
FBH - FFH
0.89
SCSI Programming Guidelines
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