Sony DDS-4 Product Description Manual page 43

5. SCSI INTERFACE
A SYNCHRONOUS DATA TRANSFER REQUEST (SDTR) message exchange shall be initiated by an SCSI device
whenever a previously-arranged data transfer agreement may have become invalid. The agreement becomes invalid
after any condition which may leave the data transfer agreement in an indeterminate state such as:
(1) after a hard reset condition
(2) after a BUS DEVICE RESET message and
(3) after a power cycle
In addition, an SCSI device may initiate an SDTR message exchange whenever it is appropriate to negotiate a new data
transfer agreement (either synchronous or asynchronous). SCSI devices that are capable of synchronous data transfers
shall not respond to an SDTR message with a MESSAGE REJECT message.
The SDTR message exchange establishes the permissible transfer periods and the REQ/ACK offsets for all logical units
and target routines on the two devices. This agreement only applies to data phases. The transfer period factor times four
is the value of the transfer period.
Transfer period is the minimum time allowed between leading edges of successive REQ pulses and of successive ACK
pulses. The SDT-10000/SDT-11000 supports the following transfer periods:
Transfer Period (hex)
Transfer Rate (MB/sec) - narrow
Transfer Rate (MB/sec) - 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 SDT-10000/SDT-11000 supports synchronous data transfer REQ/ACK offset values from 1 to 15.
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 substi-
tutes 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.
5 - 6 SONY SDT-10000/SDT-11000 DDS Tape Drive
0C 19
20.0 10.0
40.0 20.0
Table 5-4: Synchronous Data Transfer Rates
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
1F 26 2C
8.0 6.67 5.71 5.00
16.0 13.34 11.42 10.00
32 38
4.44
8.88