Rockwell Automation 2080-LC50 Micro800 Controllers: Getting Started with CIP Client Messaging - Manual (Page 56)

50 Rockwell Automation Publication 2080-QS002A-EN-E - April 2013
Appendix F Programming Tips and Recommendations for Micro800 CIP Messaging Instructions (MSG)
3. Do not use MSG instructions EN/ENO as it could leave the MSG
instruction unscanned. Normally, without the EN input, the MSG
instruction is always scanned even if input is FALSE.
For example: EN/ENO option should not be used with MSG instruction
because EN is an optimization to completely skip over the function block
if the EN input is False.
4. Execute only a single message instruction to the same server by using the
output status (for example, Q/Done) to trigger the next message
instruction. This optimizes communication performance. In case of cable
break, this limits the number of message instruction timeouts to the same
server.
Client messages are inserted into a queue when initially executed. Multiple
messages to the same server can be queued, but messages are only
transmitted one at a time.
For example, if two message instructions are executed to the same
destination server in the same program scan, the first message is sent but
the second message is delayed until the first message completes.
5. Do not have the user program take action on initial communication
timeouts. Client messages that use Ethernet may fail the first time with
error, especially after controller is powered up directly into run mode. This
is expected because when the controller powers up or after a change in the
Ethernet settings, it takes about 1 to 5 seconds (or more depends on
network settings/DHCP delay) for communications to initialize to send
the first message. Any client message activated during this period will result
in timeout error.