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Periodic Tasks Effects on Communication

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OK,  I have a program in a Control Logix 5561 controller(ver. 19.52) that every time we go online with it, the communication between the PLC and the PC running RS5000 is so slow you cannot navigate the program efficiently.

I am new to this facility but I noticed the programmer used 20 different periodic tasks to organize their program, mostly running at 30ms intervals. several are set to 10ms and some are even set to 3ms. I have gone through and found out that the short interval tasks have been overlapped 10's of millions of times, and the combined total of all scan times is over 160ms.

I have a strong feeling this could be causing our comms bog down and possibly other erratic behavior but would like a second opinion.

This machine has 2 indexing dials, with work stations that do work between indexes, Pick and Place operations, etc.  Is there even a need to be running these with periodic tasks?

All thoughts are appreciated.

Thank you

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Hi,

My recommendation for you is to analyze the program and take advantage of the Task Monitor Tool to modify the program in order to avoid task overlapping which prevents tasks from executing all the code and is a source of erratic behavior.

You should also reserve enough time for system overhead and reconfigure your CPU to utilize this time to serve either the continuous task (if there is one) or reserve this time for system tasks. The communication with your workstation will be served during this system overhead time.

See: Knowledgebase topic 54182 and Programming Manual 1756-pm005_-en-p.pdf

Most probably you will find that you can expand some task scan periods.

I may be wrong but this seems to have been programmed by someone accustomed to program Siemens who didn't check the performance of this application deep enough.

Bye
 

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Yes those scan times can limit or knock you off of online programming.  Early on, I inadvertently changed my scan time at one of Rockwell's road shows to 1ms and completely locked myself out of the PLC.  Had to take the PLC to program mode to be able to get online and change it back.  The 3ms scan time is probably the one that is killing you as there is very little time in between for comms.  You may want to see if one of your routines might be better served by an event rather than fast scan times.  Rockwell does recommend using periodic tasks, but they do need to be setup with realistic scan times.

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Thank you very much! Issue is resolved.  Reprioritizing and adjusting some of the periods cleared everything up.  For results see attached spreadsheet. 

Time slice makeup.xlsx

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