11 posts in this topic

We have a PLC that controls a diverts for cartons.  The PLC talks to a host to get the divert code and then this is used to trigger diverts through device net.   The issue we are having started back a few months.   We are getting good scans and the information is being sent to the host and back, but cartons are not diverting 100% of the time.   If we remote connect to the PLC and have the program open, the issue goes away.  When we disconnect, it issues occur again.   The PLC has a wired connection through a network switch.   Is this a problem with the PLC or with the network switch or somewhere higher up in the connection?  If it is the switch, why does the issue disappear when we are connected?    We are remote connecting with a laptop and the connection we use is wireless.

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What type of instruction are you using to trigger your divert? How long is the input trigger duration?

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We receive back a 2 digit divert code that is passed along from zone to zone to trigger the divert.

A message is received from the scanner into a string and it is modified and sent to the host.  

We have gap control timing on the PE after the scanner, 525ms to allow for adequate spacing.

 

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Without seeing your program I can only make wild guesses, but you being online with the controller does affect the controllers scan time.  Are these codes coming over explicit messaging or through mapped I/O points?  If explicit is the a corresponding input that tells you when to read the 2 digit code? What is the scan time when connected?  What type of PLC is it?

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5/05 PLC.   Ok, so why does it improve when I am connected?    When I disconnect, the issue appears.

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Again without seeing you program it is hard to tell, but if there were a timing issue such as a hard wired trigger happening before the 2 digit divert code arriving over the devicenet network or implicit vs explicit messaging, then going online would slow down your scan time slightly which could change the timing of the input vs 2 digit arrival.

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But when I go online, it improves.    I may have our network team look at the switch and how the port is configured on that switch. 

Once I find out more from them, I will add more to the post. 

Appreciate the help.

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I don't think it will have anything to do with the network switch since you data is coming over Devicenet.  If the data is coming over Devicenet then you Ethernet configuration isn't causing the problem, it is more  like it is helping expose the root problem.  See page 30 of the following manual.  Step 4 is what I believe is making it "fix itself" while you are online.  Servicing communications such is you PC affects the scan time.  Can you post your program?  I'm still leaning towards a data synchronization problem such as the trigger that starts the 526ms timer and the 2 digit divert code coming in at separate times.

http://literature.rockwellautomation.com/idc/groups/literature/documents/um/1747-um011_-en-p.pdf

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All communication from the PLC to the Host (database) is over Ethernet.  Communication from the PLC to diverts on the actual line is device net.

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that was already mentioned...;-)

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Tim is trying to convey that going online makes the PLC scan time longer. His thinking is that the divert is dependent on the timing of the communications, and that the scan time of the PLC is on the edge of being too fast, which might throw the timing off. Going online slows the PLC down enough to resolve the issue. I've seen this sort of thing before, so I agree it's pretty likely.

Ideally you can figure out what the timing issue is and correct it properly.

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