Seagate SCSI Interface Product Manual page 50

36
Responding device
SDTR response
1. Non-zero REQ/ACK offset
2. REQ/ACK offset equal to zero
3. Message Reject message
4. Parity error (on responding message)
5. Unexpected bus free (as a result of
the responding message)
6. No response from initiator to drive
initiated SDTR
If the initiator recognizes that negotiation is required, it asserts the ATN signal and sends an SDTR message to
begin the negotiating process. After successfully completing the Message Out phase, the target shall respond
with the proper SDTR message. If an abnormal condition prevents the target from returning an appropriate
response, both devices shall go to asynchronous data transfer mode for data transfers between the two
devices.
Following target response (1) above, the implied agreement for synchronous operation shall be considered to
be negated by both the initiator and the target if the initiator asserts the ATN signal and the first message out is
either Message Parity Error or Message Reject. In this case, both devices shall go to asynchronous data trans-
fer mode for data transfers between the two devices. For the Message Parity Error case, the implied agreement
shall be reinstated if a retransmittal of the second of the pair of messages is successfully accomplished. After
one retry attempt, if the target receives a Message Parity Error message, it shall terminate the retry activity.
This may be done either by changing to any other information transfer phase and transferring at least one byte
of information or by going to the Bus Free phase (see Section 3.1.1). The initiator shall accept such action as
aborting the negotiation, and both devices shall go to asynchronous data transfer mode for data transfers
between the two devices.
If the target recognizes that negotiation is required, it sends an SDTR message to the initiator. Prior to releas-
ing the ACK signal on the last byte of the SDTR message from the target, the initiator shall assert the ATN sig-
nal and respond with its SDTR message or with a Reject Message. If an abnormal condition prevents the
initiator from returning an appropriate response, both devices shall go to asynchronous data transfer mode for
data transfers between the two devices.
Following an initiator's responding SDTR message, an implied agreement for synchronous operation shall not
be considered to exist until the target leaves the Message Out Phase, indicating that the target has accepted
the negotiation. After one retry attempt, if the target has not received the initiator's responding SDTR message,
it shall go to the Bus Free phase without any further information transfer attempt (see Section 3.1.1). This indi-
cates that a catastrophic error condition has occurred. Both devices shall go to asynchronous data transfer
mode for data transfers between the two devices.
Note.
SCSI devices capable of wide data transfers (>8 bit plus parity) shall initiate a WDTR message before
initiating the SDTR message.
If, following an initiator's responding SDTR message, the target shifts to Message In phase and the first mes-
sage in is Message Reject, the implied agreement shall be considered to be negated and both devices shall go
to asynchronous data transfer mode for data transfers between the two devices.
The implied synchronous agreement shall remain in effect until a Bus Device Reset message is received, until
a hard reset condition occurs, or until one of the two SCSI devices elects to modify the agreement. The default
data transfer mode is asynchronous data transfer mode. The default data transfer mode is entered at power on,
after a Bus Device Reset message, or after a hard reset condition.
Note. Renegotiation at every selection is not recommended, since a significant performance impact is likely.
The REQ/ACK offsets supported by a particular drive are given in individual drive's Product Manual, Volume 1,
REQ/ACK offsets section.
Implied agreement
Synchronous transfer (i.e., each device transmits data with a trans-
fer period equal to or less than the values received in the other
device's SDTR message)
Asynchronous transfer
Asynchronous transfer
Asynchronous transfer
Asynchronous transfer
Asynchronous transfer
SCSI Interface Product Manual, Rev. J