Rockwell Automation 1756-L6x_L7x ControlLogix System User Manual - Manual (Page 117)

Rockwell Automation Publication 1756-UM001O-EN-P - October 2014 117
Serial Communication on 1756-L6x Controllers Chapter 6
Program the Message Instruction
Add and configure the Message instruction according to the protocol you are
using. For more information to specify the configuration details, see the
Logix5000 Controllers General Instruction Reference Manual, publication
1756-RM003 .
IMPORTANT When using structured text, broadcast over a serial port is set by typing
MSG(aMsg) and right-clicking an MSG to display the Message Configuration
dialog box.