HEEEllP

Nick Z

Member
Join Date
Jun 2015
Location
NJ
Posts
15
I have a problem with a machine that has 2 parts that each have its own plc but both possibly report to one HMI, not sure, However I keep getting a error for the laminator not communicating when I find the bit it originates from a message command that reports to a data table value in an equal command
 
I mean not to be unproductive, but how do you expect us to help you with so little information given?

What is the error?
What HMI/PLC are you using?
Is there any communication between PLC and HMI at all?
Why can't you find out if the HMI talks to both PLCs?

We love to help, but you have to understand that the only information about the machine in question is what you give us.
 
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sorry I was so brief when I re-read it, it barley makes sense to me. however isn't everybody supposed to understand whats in my head automatically typing out all of the info and my thoughts is rather laborious. All kidding aside I had two plcs with a read/write message in both setup with a heartbeat timer. Odly enough one of the timers just stopped counting. I removed all of the logic upstream of it and checked the subroutine line in the main program and it should have been timing and letting messages go every 2 seconds. It wasn't until I cycled the key switch to run that it jumped up and took off running. It was like the plc just decided to pause. I was in remote run mode during all of this happening. Sorry I wasn't more clear on the problem I was having and one of these days I hope to learn more about plc's when I'm not under fire although it seems those are the times when your knowledge always runs thin.
 
Nick Z,

From what you are describing, it sounds like a message was sent from plc(A) to a plc(B). Plc(B) either didn't receive it, or it did not acknowledge the message received back to PLC(A) and so the timer froze. There should have been some fault logic to catch the error or resend the message after x seconds.

regards,
james
 
Your description of the communications beginning to work again after a keyswitch cycle suggests that the controller is experiencing a buffer overflow.

You will probably find that the logic executes a message instruction every 2 seconds without considering whether the previous message instruction has completed successfully or unsuccessfully. In the A-B instruction sets, we call these status indications the /DN and /ER bits of the message control block.

This can work if there are never any interruptions or failures on the network, but quickly gets backed up if there's a failure or timeout or retry. The logic just keeps on triggering MSG instructions and eventually the buffer that the PLC controller is holding them in while trying to deal with the active ones that are not completing gets filled up.

If you want to investigate the issue further, please post another thread with more details about the controller, the network, and the program.
 
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