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For further details, refer to CHANGE DEFINITION parameter list (Reselection Retry, Reselection
Time-out Delay) in item 3.1.4 and SCSI Bus (RESELECTION Phase) in Chapter 1 of "SCSI
Physical Interface Specifications" and SCSI Bus Error Recovery Processing in Chapter 3.
1.3.4
Synchronous Mode Data Transfer/Wide Mode Data Transfer
The IDD is equipped with a synchronous mode data transfer function and wide mode data
transfer function for processing high speed data transfers (DATA IN and DATA OUT phases)
on the SCSI bus. Data transfers on the SCSI bus can be executed in any desired combination
of the asynchronous mode or synchronous mode and in 8 bit widths or 16 bit widths (wide
mode), but by using synchronous mode data transfer or wide mode data transfer, command
processing time is shortened and throughput for input/output processing by multiple command
processing using idle time on the SCSI bus can be improved.
Following IDD power on, after generating a RESET condition on the SCSI bus or after one of
the INITs issues a BUS DEVICE RESET message, the IDD's data transfer mode (default
mode) is the asynchronous mode. In order to use wide mode data transfer, the INIT must
exchange a SYNCHRONOUS DATA TRANSFER REQUEST message with the IDD, and the
parameters necessary for executing synchronous mode transfers must be decided. When
exchange of the WIDE DATA TRANSFER REQUEST message is completed, if the
synchronous mode has been established between the INIT and IDD previously, caution should
be exercised as to whether the asynchronous mode is set.
The INIT which uses wide mode data transfer normally sends the WIDE DATA TRANSFER
REQUEST to the IDD following the IDENTIFY message after the initial SELECTION phase,
and requests that the TARG (IDD) set the SCSI bus width in the wide mode. Also, if
synchronous mode data transfer is used, after settling the SCSI bus width, the INIT exchanges
the SYNCHRONOUS DATA TRANSFER REQUEST message and requests that the TARG
(IDD) set the synchronous data transfer mode.
The data transfer mode set with the INIT once is effective until a RESET condition is
generated or until a BUS DEVICE RESET message is issued by any one of the INITs.
Therefore, in order for the INIT to avoid overhead time for message exchange, it is not
necessary to send the WIDE DATA TRANSFER REQUEST message or the
SYNCHRONOUS DATA TRANSFER REQUEST message to the TARG each time the
SELECTION phase is executed.
When the synchronous mode transfer/wide mode data transfer request through the
specification of the CHANGE DEFINITION command (synchronous mode transfer/wide
mode data transfer request) is permitted, and the IDD is maintaining the default transfer mode
(asynchronous, 8 bit width), if a WIDE DATA TRANSFER REQUEST message is not sent
from the INIT, the IDD enters the MESSAGE IN phase immediately after the COMMAND
phase and sends the WIDE DATA TRANSFER REQUEST message to the INIT, and tests the
16 bit wide mode setting.
After establishing the bus width, the IDD sends the
SYNCHRONOUS DATA TRANSFER REQUEST message to the INIT and tests the
synchronous mode transfer parameters (REQ/ACK offset = 15, Transfer period = 100 ns).
The IDD maintains separate data transfer mode settings between itself and each INIT.
Therefore, an INIT which uses asynchronous mode transfer and an INIT which uses
synchronous mode transfer can both coexist on the same SCSI bus. The parameters for
synchronous mode transfers decided by the SYNCHRONOUS DATA TRANSFER REQUEST
message can differ for each INIT and an INIT which uses the 8 bit wide transfer mode can
coexist with an INIT which uses the 16 bit wide transfer mode.
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