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Old January 2nd, 2018, 12:01 PM   #1
bodoo23
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RPI & Periodic Event

I am new in programming. There are two things I am wondering about

1- if we set RPI of a module 20ms and if the scan time is for example 70ms what would happen ? PLC will get values from module in the beginning and process these values in logic but what happen after 20ms when plc get new values from module in the same time scan is going on ? Can it use these values or it will ignore second (2x20ms) values and will use third(3x20ms) values in the second scan(after 70ms)

2-The second question is similar actually. when the period of periodic task is 10ms and scan time is 25ms. are the inputs and outputs used in periodic task being updated in every 10ms or it is like other values and need to be wait for the scan to be completed ? if they are like the values used in continuous tasks and just to be updated before and after scans. What are the advantageous of using periodic tasks ?

I hope my questions were clear since I am not very good at english I might not clarify well. Thanks
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Old January 2nd, 2018, 12:42 PM   #2
RonJohn
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Welcome to the forum bodoo23! Let me see if I understand correctly. You didn't specify which processor you are using, so I will discuss from the perspective of an A-B Logix based controller (other types may differ).

1. I'm not sure what you mean by scan time - it sounds like you're saying the program takes 70ms to complete a scan. In that case, the data will get updated every 20ms (based on RPI). There is no guarantee when the data will update unless you choose to buffer the input data. For example, I buffer my data at the beginning of each program scan and only use the buffer tags throughout my program. This ensures that the input data is not changing somewhere mid-scan.

2. The advantage of using a periodic task rather than a continuous task is that you know exactly how often the task will execute. Also, continuous task scan time will increase as the size of the program increases but a 10ms periodic task will execute every 10ms even as program size increases. Now if you have a 10ms task that takes 25ms to scan, this will create an error. Periodic task time must be greater than the actual task scan time.

For a good discussion of RPI, RTS and other such settings, check out THIS thread from the forum a few years ago
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Old January 2nd, 2018, 12:52 PM   #3
bodoo23
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Hi RonJohn. Thanks for answers. As I have known by now PLC first read inputs then process logic and write the outputs and the input values cant be updated in process of logic(It will remain the same value for this cycle even if the value changes during this scan). So now I see my understanding was wrong. If it changes in the mid of scan, it is updated immediately even if it is a physical input ?
Thanks again.
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Old January 2nd, 2018, 01:00 PM   #4
Ron Beaufort
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this thread might also be helpful to you ...

http://www.plctalk.net/qanda/showthr...7&postcount=61

it gives some simple "test" programs that you can experiment with - so that you can discover how these "update" features work on your own ...

welcome to the forum - and your English is perfectly understandable ...
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Old January 2nd, 2018, 01:01 PM   #5
RonJohn
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Quote:
Originally Posted by bodoo23 View Post
If it changes in the mid of scan, it is updated immediately even if it is a physical input ?
This depends on which controller you are using. **If you tell us what you're working with, we can give you a more clear answer.

As I stated initially, my example used an A-B Logix based controller. In these, the inputs update asynchronously to program scan. With an A-B SLC500 or PLC5, the IO updates synchronously at the end of each program scan. For the SLC or PLC5, input buffering was not necessary.
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Old January 2nd, 2018, 01:05 PM   #6
bodoo23
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Quote:
Originally Posted by RonJohn View Post
This depends on which controller you are using. **If you tell us what you're working with, we can give you a more clear answer.

As I stated initially, my example used an A-B Logix based controller. In these, the inputs update asynchronously to program scan. With an A-B SLC500 or PLC5, the IO updates synchronously at the end of each program scan. For the SLC or PLC5, input buffering was not necessary.
I am working with 1756-L62 plc.
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Old January 2nd, 2018, 01:14 PM   #7
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OK, so a 1756-L62 will scan the IO asynchronous to (not necessarily in step with) the program scan since that is a ControlLogix processor. For many of us who started our careers with SLC500 or PLC5, this was an adjustment for us also.

I agree with Ron Beaufort that your English is good - much better than my Turkish! Let me encourage you to use the forum search for a lot of useful information and feel free to post questions as we are all learning.
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Old January 2nd, 2018, 01:28 PM   #8
bodoo23
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Thank you both for answers & kindness. I am really eager to learn and now I feel like I am talking with my friends. It will make me feel free to ask questions. I will keep searching here.
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