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1.3 Outline of Command Processing
C141-E167 1 - 15
3) After the INIT that accepts the IDENTIFY message normally completes the pointer
restore operation, it should make the ACK signal for the IDENTIFY message FALSE.
If the ATTENTION condition does not exist when the ACK signal becomes FALSE
during sending of the IDENTIFY message, the IDD regards the reconnection
processing as having been normally completed and begins subsequent processing.
For further details, refer to CHANGE DEFINITION parameter list (Reselection Retry,
Reselection Time-out Delay) in Section 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 equips 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.
After IDD power on, after generating a RESET condition on the SCSI bus or after one of the INITs
issues a TARGET RESET message, the data transfer mode (default mode) of the IDD is the
asynchronous mode. To use wide mode data transfer, the INIT must exchange a WIDE DATA
TRANSFER REQUEST message or PARALLEL PROTOCOL REQUEST message with the IDD,
and determine the SCSI bus width. To use the synchronous mode data transfer, the INIT must
exchange the SYNCHRONOUS DATA TRANSFER REQUEST message or PARALLEL
PROTOCOL REQUEST message and determine parameters necessary for executing the synchronous
mode transfer. When exchange of the WIDE DATA TRANSFER REQUEST message or
PARALLEL PROTOCOL REQUEST message is completed, if the synchronous mode has been
established between the INIT and IDD previously, note that the asynchronous mode is set.
The INIT which uses wide mode data transfer normally sends the WIDE DATA TRANSFER
REQUEST or PARALLEL PROTOCOL REQUEST message to the IDD following the IDENTIFY
message after the initial SELECTION phase, and requests the TARG (IDD) to set the SCSI bus width
in the wide mode. Also, if synchronous mode data transfer is used, after establishing the SCSI bus
width, the INIT exchanges the SYNCHRONOUS DATA TRANSFER REQUEST message or
PARALLEL PROTOCOL REQUEST message and requests the TARG (IDD) to set the synchronous
data transfer mode.
The data transfer mode set with the INIT once is effective until a RESET condition occurs or until a
TARGET RESET message is issued by any INITs. Therefore, in order for the INIT to avoid
overhead time for message exchange, the INIT should not send the WIDE DATA TRANSFER
REQUEST message and the SYNCHRONOUS DATA TRANSFER REQUEST message or
PARALLEL PROTOCOL REQUEST message to the TARG each time the SELECTION phase is
executed.
When the requesting synchronous mode transfer/wide mode data transfer by specifying 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 for attempting to set the 16-bit width mode. After
establishing the bus width, the IDD sends the SYNCHRONOUS DATA TRANSFER REQUEST
message to the INIT and attempts the synchronous mode transfer parameters (REQ/ACK offset =
127, Transfer period = 25 ns).