Rockwell Automation Logix5000 Controllers Messages Programming Manual - Manual (Page 13)

Controller messages Chapter 1
Term Definition
The controller can have 10 to 40 unconnected buffers.
•
The default number is 10.
• To increase the number of unconnected buffers, execute a MSG instruction that reconfigures the
number of unconnected buffers.
• Each unconnected buffer uses 1.2 KB of memory.
• If all unconnected buffers are in use when an instruction leaves the message queue, an error
occurs and data does not transfer.
If a MSG instruction uses a connection, the instruction uses an unconnected
buffer when it first executes to establish a connection. If you configure the
instruction to cache the connection, it no longer requires an unconnected buffer
once the connection is established.
As you plan and program your MSG instructions, follow these guidelines.
Guideline Details
For each MSG instruction, create a 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 access only tags that are in the Controller Tags
folder (controller scope).
If your message is to a device 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.
Logix5000 controllers execute more efficiently and use less memory when
working with 32-bit integers (DINTs).
See "
Convert between INTs and DINTs (on page 16 )”.
Cache the connection for those MSG
instructions that execute most frequently, up
to the maximum number permissible for your
controller revision.
Execution time is optimized when the controller does not open a
connection each time the message executes.
If you want to enable more than 16 MSGs at
one time, use a management strategy to
ensure some MSG instructions are not
delayed entering the queue.
To guarantee the execution of each message, use one of these options:
•
Enable each message in sequence.
• Enable the messages in groups.
• Program a message to communicate with multiple devices.
• 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 to 40 unconnected buffers. The default number
is 10.
•
If all unconnected buffers are in use when an instruction leaves the
message queue, an error occurs, the data is not transferred.
• You can increase the number of unconnected buffers (up to 40),
provided you continue to adhere to the previous guideline.
• To increase the number of unconnected buffers, see “ Get or set the
number of unconnected buffers (on page 14 )”.
Guidelines
Rockwell Automation Publication 1756-PM012F-EN-P - October 2014 13