Indexing Source and Target address in the PLC5 MSG Instruction

Join Date
May 2002
Posts
4
Hi,

I'm trying to use only one Msg Instruction where the Source and the Target addresses are Indexed (Dynamic), i made that using an Integer (N) file address in the Control of the MSG instead of using the (MG) file address.
The code work's fine for the indexation but i still have a problem:

How can i define the Port Address for the DH+ network using the (N) file address ?
Maybe by default it's not possible to send messages out through other port than channel 1A.

Can somebody help me ?

Thank's for your time,

Walter Moreira
 
From RSLogix 5 Help file:

MSG - Message Instruction

Using the MG control block, the control block size is fixed at 56 words. You must use the MG control block if you are sending messages to an SLC 500 processor using the SLC read and write commands, or if you are sending messages out any other port than channel 1A.

© 1997 Rockwell Software Inc.

Why are you trying to do this? Why not just create all the possible MSG instructions you need, and enable only those you want? I'm fairly sure that Rockwell warns agains indirectly (or indexedly) addressing data in MSG blocks - although they don't say why.

They do say:
You cannot use indirect addresses for the control block address in an MSG instruction. If you have an MSG instruction created with version 3.21 or earlier that uses a control block with an indirect address, you must delete the instruction and re-enter it without an indirect address.

- Instruction set manual, p. 16-3

Remember that the MSG is asynchonous to scan (although it may be triggered synchronous to scan). Changing parameters while the MSG is still "in motion" could bring trouble.
 

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