Allen-Bradley MicroLogix 1200 Reference Manual page 302

21-2 Communications Instructions
Publication 1762-RM001C-EN-P
storage area that keeps track of messages that have not been allocated a
buffer. The queue operates as a first-in first-out (FIFO) storage area. The
first message request stored in the queue is the message that is allocated a
buffer as soon as a buffer becomes available. The queue can
accommodate all MSG instructions in a ladder program.
When a message request in a buffer is completed, the buffer is released
back to the system. If a message is in the queue, that message is then
allocated a buffer. At that time, the data associated with the message is
read from within the controller.
If a message instruction was in the queue, the data that is
NOTE
actually sent out of the controller may be different than
what was present when the message instruction was first
processed.
The buffer and queue mechanisms are completely automatic. Buffers are
allocated and released as the need arises, and message queuing occurs if
buffers are full.
The controller initiates read and write messages through available
communication channels when configured for the following protocols:
• DH-485
• DF1 Full-Duplex
• DF1 Half-Duplex Slave
For a description of valid communication protocols, see Protocol
Configuration on pag eE-1.