Sony DDS-4, Руководство по описанию продукции

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5 - 8 SONY SDT-10000/SDT-11000 DDS Tape Drive
5. SCSI INTERFACE
In addition, an SCSI device may initiate a WDTR message exchange whenever it is appropriate to negotiate a new
transfer width agreement. SCSI devices that are capable of wide data transfers (greater than eight bits) shall not respond
to a WDTR with a MESSAGE REJECT message.
The WDTR message exchange establishes an agreement between two SCSI devices on the width of the data path to be
used for DATA phase transfers between the two devices. This agreement applies to DATA IN and DATA IN and DATA
OUT phase only. All other information transfer phase shall use an eight-bit data path.
If an SCSI device implements both wide data transfer option and synchronous data transfer option, then it shall negotiate
the wide data transfer agreement prior to negotiating the synchronous data transfer agreement. If a synchronous data
transfer agreement is in effect, then an SCSI device that accepts a WDTR message shall reset the synchronous agree-
ment to asynchronous mode.
The transfer width is two to the transfer width exponent bytes wide. The transfer width that is established applies to an
logical units on both SCSI devices. Valid transfer widths are 8 bits (m=00h), and 16 bits (m=01h). Values of m=02 and
greater are reserved.
The originating SCSI device (the SCSI device that sends the first of the pair of WDTR messages) sets its transfer width
value to maximum data path width it elects to accommodate. If the responding SCSI device can also accommodate this
transfer width, it returns the same value in its WDTR message. If it requires a smaller transfer width, it substitutes the
smaller value in its WDTR message. The successful completion of an exchange of WDTR messages implies an agree-
ment as follows:
Responding Device WDTR response Implied Agreement
a) Non-ZERO transfer width Each device transmits and receives data with a transfer width equal to the
responding SCSI device’s transfer width.
b) Transfer width equal to ZERO Eight-bit data transfer
c) MESSAGE REJECT message Eight-bit data transfer
If the initiator recognizes that negotiation is required, it asserts the ATN signal and sends a WDTR message to begin the
negotiating process. After successfully completing the MESSAGE OUT phase, the target shall respond with the proper
WDTR message. If an abnormal condition prevents the target from returning an appropriate response, both devices shall
go to eight-bit data transfer mode for data transfers between the two devices.
Following target response (1) above, the implied agreement for wide data transfers shall be considered to be negated by
both the initiator and the target if it the initiator asserts ATN and the first message out is either MESSAGE PARITY
ERROR or MESSAGE REJECT. In this case, both devices shall go to eight-bit 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 a vendor-specific number of retry
attempts (greater than ZERO), 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 on 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 eight-bit data transfer mode for data transfers between the two devices.