Logix5000 Controllers General Instructions - SLAC Confluence

140 Input/Output Instructions (MSG, GSV, SSV, IOT)Message (MSG)The MSG instruction asynchronously reads or writes a block of data toanother module on a network.Operands:Relay LadderOperand Type Format DescriptionMessage MESSAGE tag message structurecontrolStructured TextMSG(MessageControl);The operands are the same as those for the relay ladderMSG instruction.MESSAGE StructureATTENTIONIf you check the status bits more than onceThe controller changes the DN, ER, EW, and ST bits asynchronous to the scan of your logic.Use a copy of the bits if you check them in more than one place in your logic. Otherwise, thebits may change during the scan and your logic won’t work as you expect it.One way to make a copy is to use the FLAGS word. Copy the FLAGS word to another tag andcheck the bits in the copy.IMPORTANTDo not change the following status bits of a MSG instruction:• DN• EN• ER• EW• STDo not change those bits either by themselves or as part of the FLAGS word. If you do, thecontroller may have a non-recoverable fault. The controller clears the project from itsmemory when it has a non-recoverable fault.Publication 1756-RM003I-EN-P - January 2007