Rockwell Automation Logix5000 Controllers Messages Programming Manual - Manual (Page 10)

Chapter 1 Controller messages
This diagram shows how the controller processes MSG instructions.
Description
The controller scans the MSG instruction and its rung-condition-in goes true. The message passes to a throttle that
has 16 positions. If the throttle is full, the message remains enabled but is held until another controller scan.
The System-overhead time slice executes and the message is pulled from the throttle to the message queue.
If the MSG instruction Then the MSG instruction
Does not use a connection or the connection was not
previously cached Uses an unconnected buffer to establish
communication with the destination device.
Uses a connection and the connection is cached Does not use an unconnected buffer.
Communication occurs with the destination device.
The message queue holds up to 48 MSG instructions, including those that you
configure as a block-transfer read or block-transfer write. When the queue is full,
an instruction tries to enter the queue on each subsequent scan of the instruction,
as shown in the following illustration.
Message queue
10 Rockwell Automation Publication 1756-PM012F-EN-P - October 2014