CLogix with 1756 DHRIO Card controlling PLC5 IO

Latzi

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Join Date
Nov 2007
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Hi Gents,

I just finished the first stage of converting a PLC 5 system with 1 local rack + 1 remote rack to a Controllogix L61 system. I did convert the software and re mapped the IO accordingly and it's all good. However.
There are a couple of counters in the user program which simply count loaves of bread on a conveyor. Those counters count up every time a physical input is made. The input cannot do more than three on/off cycles / second.
It appears from the feedback I had from operators that these counters don't work properly as in they are missing counts.3 Hz is not a high frequency by any standard and the 1771 ASB and the DHRIO should be able to handle this easily. Not sure what should I look at.
I took a screenshot of the Controller Organizer window and attached this. I know that I didn't provide enough info but am happy to do it if someone has any idea why would I have this issue.

Thanks

IO_Config.jpg
 
Are the input signals true and false for equal amounts of time, or are they "pulses" that are relatively infrequent ?

RIO can update at about 10 ms per logical Rack at 57.6 kb/s, so your four logical Racks are taking about 40 ms total to scan.

There's a default RPI setting for each Adapter that exchanges data between the 1756-DHRIO and the ControlLogix CPU across the ControlLogix backplane. By default this is set to 48 ms.

If the two chassis are less than 2500 cable-feet from the 1756-DHRIO, you should also change the RIO data rate to 230,400 baud instead of 57600. Then you could cut the RPI value for each Adapter down to 10 ms, and have a much better I/O response time.
 
From the IO Tree that you have provided, it appears that you have some Analog IO as well.

There is an issue with ControlLogix communicating to 1771 Analog IO. It has to do with the ASB not being able to handle more than a single MSG to read or write to the IO module. If you have more than one analog module in a chassis, then you would want to utilize a message manager that steps through your Analog IO modules.

I recently completed retrofitting a system that had ControlLogix processors communicating to 7 chassis of IO via DHRIO modules. 3 of these were dedicated to analog IO. Problem we had was the response time of the process data. Therefore, I retrofitted the system with 1756 IO utilizing ControlNet protocol. Went from approximately 5 second analog update rate to approximately 100 ms.

Point of this is that just because you can, doesn't mean you should. If it may help, AB does have a method of using the 1771 Wiring arms with 1756 IO modules. This could aid if re-wiring is not desired. I did not use this method for other reasons.
 
Hi Gents,


Thanks for your answers.The input signals are totally infrequent and the maximum frequency is actually 2.3 Hz. I will try however to increase the baud and drop the RPI which is set to 48 right now. See what effect that will have.
Regarding the analog signals I am not too worried about as they are non process critical and I did slow the MSG down to once every 5 seconds.
Thank you again and I'll update what effect the baud increase ,rpi decrease had.
 
Forgot to say that the IO will be upgraded as well in two months time.This is only a transitional stage ,I just wont to make sure that the plant operates as it used to before starting the upgrade.
Thanks
 
Forgot to say that the IO will be upgraded as well in two months time.This is only a transitional stage ,I just wont to make sure that the plant operates as it used to before starting the upgrade.
Thanks

Before you go looking for the weird and wonderful, has there been any history before the conversion counts were missed?
Sometimes operators and management expect a controls upgrade to solve sensor issues.
 
Before you go looking for the weird and wonderful, has there been any history before the conversion counts were missed?
Sometimes operators and management expect a controls upgrade to solve sensor issues.
It was the very first question I did ask operators... Is this something new or it was an issue before the conversion.
 
I couldn't yet lower the RPI on both the DHRIO and 1771 ASB modules. At this stage I think I am not going to increase the baud . 57.6k should be enough if I can lower the RPI of the DHRIO card and the ASB adapter to 10 ms the same like the old PLC CPU scan time used to be.
 

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