5.SCSI Interface
SONY AIT-5 drive SDX-1100V series Ver.1.0 5-7
Table 5-4: Synchronous Data Transfer Rates
Transfer Period (hex)
9
0A
0C
19
1F
26
2C 32 38
Transfer Rate (MB/s) - narrow
80
40.0
20.0
10.0
8.0 6.67 5.71 5.00 4.44
Transfer Rate (MB/s)- wide
160
80.0
40.0
20.0
16.0
13.34 11.42 10.00 8.88
REQ/ACK offset: is the maximum number of REQ pulses allowed to be outstanding before the leading edge of its
corresponding ACK pulse is received at the target. A REQ/ACK offset value of ZERO shall indicate asynchronous
data transfer mode. The SDX-1100V supports synchronous data transfer REQ/ACK offset values from 1 to 127.
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
successfully with these 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, it 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 messages 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
If the initiator recognizes that negotiation is required, it asserts the ATN signal and sends a 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
transfer mode for data transfers between the two devices. For the MESSAGE PARITY ERROR case, the implied
agreement shall be reinstated if a re-transmittal of the second of the pair of messages is successfully
accomplished. After two retry attempts, 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. The initiator shall accept such
action as aborting the negotiation, and both devices shall go to asynchronous data transfer mode for data transfer
between the two devices.
If the target recognizes that negotiation is required, it sends an SDTR message to the initiator. Prior to releasing
the ACK signal on the last byte of the SDTR message from the target, the initiator shall assert the ATN signal and
respond with its SDTR message or with a MESSAGE 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 two retry attempts, 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. This indicates that a catastrophic
error condition has occurred. Both devices shall go to asynchronous data transfer mode for data transfer between
the two devices.
If, following an initiator's responding SDTR message, the target shifts to MESSAGE IN phase and the first
message 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.
