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4.6 Messages
This section describes the detection of an ATN signal and explains the types of
messages supported by this scanner.
4.6.1 ATN detection
The following table summarizes the timing at which this scanner detects an ATN
signal.
Phase ATN detection timing
SELECTION Immediately before a phase change
COMMAND Immediately before a phase change
DATA OUT Immediately before a phase change
DATA IN Immediately before a phase change (*1)
STATUS Immediately before a phase change
MESSAGE OUT Upon each reception of a message
MESSAGE IN Upon each transmission of a message
*1 This scanner detects ATU signal for every Read command during an image
data transmission. When an ATN signal is detected, scanning operation is
interrupted.
4.6.2 Message types
Messages provide information consisting of one or more bytes that are transferred
in the MESSAGE IN and MESSAGE OUT phases. These messages are used to
control the bus phase sequence.
The initiator creates an ATTENTION condition for the target, indicating that it
has a message to be reported to the target. Only then the target switches to the
MESSAGE OUT phase to receive the message.
If the target has completed the SELECTION/RESELECTION phase, it can execute
the MESSAGE IN phase at any time to send the message to the initiator.
The messages that can be communicated with this scanner are listed in the
following table.
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