Sony SDX-900V Product Specifications page 51

Transfer Period (hex)
Transfer Rate (MB/s) - narrow
Transfer Rate (MB/s)- wide
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-900V 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
(1) Non-ZERO REQ/ACK offset
(2) REQ/ACK offset equal to ZERO
(3) MESSAGE REJECT message
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 tow 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.
SONY AIT-4 drive SDX-900V series Ver.2.1
Table 5-4: Synchronous Data Transfer Rates
9 0A 0C
80 40.0 20.0
160 80.0 40.0
Implied Agreement
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.
Asynchronous transfer
Asynchronous transfer
19 1F 26 2C
10.0 8.0 6.67 5.71
20.0 16.0 13.34 11.42 10.00
5.SCSI Interface
32 38
5.00 4.44
8.88
5-7