Rockwell Automation 2706-MB1 MESSAGEVIEW 421 SOFTWARE MANUAL - Manual (Page 59)

4–11 Planning an Application
Publication 2706-817
The Message Queue
Queuing is important in situations where multiple message triggers
are likely to occur. Value triggered messages that are triggered but
cannot be displayed at once are stored in the terminal’s Message
Queue if the Queue is enabled.
Note: A bit–triggered message is not queued. The bit remains set
until the controller resets it.
The MessageView terminal supports two fixed Message Queue
lengths, set in the Terminal Setup Advanced dialog. See Page 10–13.
The length may be:
•
64 messages deep (the Queue is enabled)
•
1 message deep (in effect, the Queue is disabled)
The MessageView terminal processes messages according to the
following criteria:
•
A triggered message is displayed at once if possible.
•
If the specified line for a value–triggered message is not free, it is
placed at the bottom of the Message Queue Stack.
•
Messages are processed from the Queue Stack in a top-to-bottom
First In, First Out (FIFO) priority.
•
The topmost message is displayed when its assigned display line
is available. The terminal continues to queue additional triggered
messages while it waits to display the topmost message.
Note: Messages in the Queue are not displayed until they reach
the top of the Queue, even if their line is available.
•
If the Queue reaches Full status, the next message that is queued
will be entered at the top of the Queue, overwriting a pre–existing
queued message.
Note: The Message Queue is not cleared when the terminal operator
enters the Front Panel Editor (FPE). When the operator closes the
FPE, Run mode resumes and the queued messages are displayed as if
there was no interruption.
The Message Queue is cleared if:
•
the terminal has processed all queued messages
•
the terminal is reset
•
the controller triggers a Clear Queue (9902) Special Message
•
the controller triggers a Terminal Reset (9903) Special Message.