Seagate SCSI Interface Product Manual page 49

SCSI Interface Product Manual, Rev. J
Table 7: Synchronous Data Transfer Request message format
Byte Value
0 01h
1 03h
2 01h
3 [3]
4 [2]
Notes.
[1] See tables in the individual drive's Product Manual, Volume 1, synchronous data transfer periods sup-
ported section, for a list of transfer periods supported by the drive described therein.
[2] The value supported by a particular drive model is listed in the individual drive's Product Manual, Volume
1, REQ/ACK offset section.
[3] Transfer factors for this field are listed in Table 8.
Table 8: Transfer Period Factor field
Code Description
00h-09h Reserved [1]
0Ah Transfer period equals 25 ns [2]
0Bh Transfer period equals 30,3 ns [2]
0Ch Transfer period equals 50 ns [3]
0Dh-18h Transfer period equals the transfer period factor * 4 [3]
19h-31h Transfer period equals the transfer period factor * 4 [4]
32h-FFh Transfer period equals the transfer period factor * 4 [5]
Notes.
[1] Faster timings may be allowed by future SCSI parallel interface standards.
[2] Fast-40 data transfer rates that have a period = 25 ns or 30,3 ns.
[3] Fast-20 data transfer rates that have a period of <96 ns and >50 ns.
[4] Fast-10 data transfer rates that have a period of <196 ns and >100 ns.
[5] Fast-5 data transfer rates that have a period of <1020 ns and >200 ns.
In addition, a 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 synchro-
nous data transfers shall not respond to an SDTR message with a Message Reject message.
The SDTR message exchange establishes the permissible transfer periods and REQ/ACK offsets for all logical
units on the two devices. This agreement only applies to Data In and Data Out phases. All other phases shall
use asynchronous transfers.
The transfer period is the minimum time allowed between leading edges of successive REQ pulses and of suc-
cessive ACK pulses to meet the device requirements for successful reception of data.
The REQ/ACK offset is the maximum number of REQ assertions allowed to be outstanding before the leading
edge of its corresponding ACK assertion is received at the target. This value is chosen to prevent overflow con-
ditions in the device's reception buffer and offset counter. A REQ/ACK offset value of zero shall indicate asyn-
chronous data transfer mode and that the transfer period factor shall be ignored. A value of FFh shall indicate
unlimited REQ/ACK offset. The REQ/ACK offset value supported by each drive is listed in each individual
drive's Product Manual, Volume 1, REQ/ACK offset section.
The originating device (the device that sends the first of the pair of SDTR messages) sets its values according
to the rules above to permit it to receive data successfully. If the responding device can also receive data suc-
cessfully with these values, it returns the same values in its SDTR message. If it requires a larger transfer
period, 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 transmit-
ting data shall respect the limits set by the other's SDTR message but it is permitted to transfer data with larger
transfer periods, smaller REQ/ACK offsets, or both than specified in the other's SDTR message. The success-
ful completion of an exchange of SDTR messages implies an agreement as follows:
Description
Extended message
Extended message length
Synchronous Data Transfer Request code
Transfer period factor [1]
REQ/ACK offset
35