Message Instruction

CallumMacEwen

Member
Join Date
Jul 2015
Location
Scotland
Posts
42
I have a problem with a message instruction. I will explain what the situation is.

In this factory we have several PLCs controlling different types of machinery. We have a master PLC that we use for gathering data from the other PLCs.

They are complaining that the time stamps are not matching up on the SCADA. We think this is happening because if a panel is switched off then the message instruction will go into error. Could this be the issue that when the message is in error its not moving on to the next stage message instruction because the one that is in error is hogging the communication.

I will enclose pictures of the message program logic.

msgcalls.jpg msgtiming.jpg
 
CallumMacEwen,

Can you please elaborate on the time stamps not matching up? It appears that your timer updates every 500ms regardless of message condition. However there are only so many messages that can be handled in the buffer at a time, so switching off a panel may cause grief.

If you haven't done so yet, download a copy of the Logix5000 Controllers Messages manual as it helps with setting up multiple messages and avoiding pitfalls.

Depending on what you're messaging from/to, you may be able to use a GSV instruction to check comm status and disable MSG instructions to modules that aren't running.

I see you're sending/receiving a MSG every 500 mS. Please insert a rung to reset MSGs either MSG ends DN or with ER.

I agree that resetting is a good idea, but you'll need to one shot the MSG instruction or else the MSG may keep executing repeatedly until the pointer advances.
 

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