Best logic for MSG Instruction in Allen Bradley

lunenburger

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Jul 2008
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Just want to put the question out to the PLC guru's...

What is the best permissive logic for an Allen Bradley PLC MSG logic?
I have seen message enables used, timers, first scan bits etc....

Is there a proven best way to always push a message through and optimize communications?

Also, what is the best way to watchdog a message?
Again, nearly ever PLC here has a different way to watchdog...
What is the best way to make sure a message hasn't failed?

Thanks
 
Unfortunately there isn't an easy answer . There are a number of different ways of handling messages ' as you have found ,which depend circumstances such as :-
Message use
Type of comms network
How busy / reliable is comms network

You will need to give more details to narrow the choices for your application .

Paul
 
Definitely all depends on your hardware and what you're trying to accomplish via the message commands.


For a watchdog, there is an internal timeout value and status for the MSG instruction (which I believe you can set to something other than default). I monitor those to know there isn't a message failure.
 
The way I handle MSGs on the ML1400 for Modbus comms with several different devices is as follows:

I use an index variable that corresponds to the different MSGs
When the index EQU the MSG's number, the MSG is engaged
The DN or ER bits of that MSG will increment the index variable
Reset index variable to 0 if index is incremented past the number of MSGs
MOV the index variable to another variable every scan (basically current index -> last index)
EQU index last_index + TON -> watchdog timer
timer.DN -> increment index

Idea is that if the index stays the same for too long, that means the message has timed out or something else went wrong, so skip it and go to the next message. The timer gets reset every time the index value changes.

Edit: I have to do it this way because the 1-second timeout built-in to MSG is way too long. With modern Modbus speeds, you can be sure of a timeout if it's been 400 ms most of the time.
 
+1 for Epy
That is very typical when using many MSG's. And it's kinda nice because you know for sure the MSG's are getting executed sequentially, one at a time.
 
We are messaging between various Allen Bradley PLC's over ethernet.
Mostly interlock between conveyors across PLC's nothing has to be very high speed.
 
If there are not too many signals, I condition my MSG instructions with a change of state of those signals,this is a more practical way of doing it if you have a busy highway when additional MSG resourses causes comms slowdown.

Steve
 

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