Rockwell Automation 1734-AENTR EtherNet/IP Network Configuration User Manual - Manual (Page 73)

Rockwell Automation Publication ENET-UM001L-EN-P - March 2014 73
Interlocking and Data Transfer between Controllers Chapter 6
Guidelines for Message (MSG)
Instructions Follow these guidelines.
For more information on programming MSG instructions, see the Logix5000
Controller General Instructions Reference Manual, publication
1756-RM003 .
The individual system user manuals for Logix5000 controllers also provide MSG
examples unique to specific controller platforms.
Table 14 - MSG Instruction Guidelines
Guideline Description
For each MSG instruction, create a
control tag. Each MSG instruction requires its own control tag:
• Data type = MESSAGE
• Scope = controller
• The tag cannot be part of an array or a user-defined data type.
Keep the source and destination
data at the controller scope. A MSG instruction can only access tags that are in the Controller Tags folder.
If your MSG is to a module that uses
16-bit integers, use a buffer of INTs
in the MSG and DINTs throughout
the project. If your message is to a module that uses 16-bit integers, such as a PLC-5 or SLC
500 controller, and it transfers integers (not REALs), use a buffer of INTs in the
message and DINTs throughout the project.
This increases the efficiency of your project because Logix5000 controllers
execute more efficiently and use less memory when working with 32-bit
integers (DINTs).
Cache the connected MSGs that
execute most frequently. Cache the connection for those MSG instructions that execute most frequently,
up to the maximum number permissible for your controller revision.
This optimizes execution time because the controller does not have to open a
connection each time the message executes.
If you want to enable more than 16
MSGs at one time, use some type of
management strategy. If you enable more than 16 MSGs at one time, some MSG instructions may
experience delays in entering the queue. To guarantee the execution of each
message, perform one of these tasks:
• Enable each message in sequence.
• Enable the messages in groups.
• Program a message to communicate with multiple modules.
• Program logic to coordinate the execution of messages.
Keep the number of unconnected
and uncached MSGs less than the
number of unconnected buffers. The controller can have 10...40 unconnected buffers. The default number is 10.
• If all the unconnected buffers are in use when an instruction leaves the
message queue, the instruction errors and does not transfer the data.
• You can increase the number of unconnected buffers to a maximum of 40.