The originating device (the device that sends the first of the pair of SDTR message) sets its values according to the rules above to permit it to receive data successfully. If the responding device can also receive data successfully with those values (or smaller transfer periods or larger REQ/ACK offsets or both), it returns the same values in its SDTR message. If it requires a larger transfer period, a smaller REQ/

ACK offset, or both in order to receive data successfully, itINTERFACE substitutes values in its SDTR message as required, returning

unchanged any value not required to be changed. Each device when transmitting 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 successful completion of an exchange of SDTR message implies an agreement as follows:

Responding Device SDTR response

 

Implied Agreement

1)

Non-zero REQ/ACK offset

Each device transmits data with a

 

 

 

transfer period equal to or greater

 

 

 

than and a REQ/ACK offset equal

 

 

 

to or less than the values received in

 

 

 

the other device's SDTR message.

2)

REQ/ACK offset equal to zero

Asynchronous transfer

3)

MESSAGE REJECT message

Asynchronous transfer

See ANSI SCSI-2 6.6.21 for details of SYNCHRONOUS

TRANSFER REQUEST message.

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