Communication instructions Chapter 9
Rockwell Automation Publication 2080-RM001D-EN-E - February 2015 217
Com queue: Message requests added to the Com queue have a buffer allocated
and processed by the communication task. The maximum queue size limit is 4.
Wait queue: Messages that cannot be added to the Com queue are added to the
Wait queue to be processed at a later time. The Wait queue does not have a
maximum size limit.
Message execution timing diagram (Rung = TRUE)
The following table describes the message conditions and bit status for each
execution step identified in the timing diagram while the rung condition remains
true.
Step Message description Bit status
1
Rung condition goes TRUE.
Message execution is enabled.
EN bit is set.
All other bits are cleared.
2
Message control buffer is acquired. At this time, input data (that is, the "data"
parameter for write messages) is copied for transmission. Subsequent changes to
the input data will not be reflected in the transmitted message.
EW bit is set.
3
Message transmission starts. EW bit is cleared.
ST bit is set.
4
Message response is received. ST bit is cleared.
DN bit is set.
5
Rung condition goes FALSE. EN bit is cleared.
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