PLC Interview Question - Array FIFO Logic Programming Example Average Rolling Mean RSLogix 5000

PLC Interview Question - Array FIFO Logic Programming Example Average Rolling Mean RSLogix 5000
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PLC Interviews aren't very complex. Within a short period of time, you may be asked to implement a simple logical structure such as a Motor Starter, a stack light system or a basic FIFO set of rungs. By practicing implementing such structure, you will build your knowledge of PLCs, the software package of your choice and you'll become a more proficient programmer.
Today, we're looking at an example that involves a FIFO Implementation and results in a way to store data based on short periods in order to compute a rolling average of such data. In other words, the logic can be used to report a rolling average of the number of products made in the last hour. At first glance, it may seem that such systems already exist, but from our experience, most will report the quantity produced at every hour. The nuance here is that as you walk up to the production line at 7:43 AM, you should be able to see how many cases were made from 6:43 AM to 7:43 AM. A traditional system will give you the number of cases from 6 AM to 7 AM and sometimes a rolling number starting from 7 AM.
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[@davesoutdooradventures5523]

Thanks for showing how to use these commands, can you give examples of applications they they might be used in please

[@ManuelMartinez-fx6ql]

Hi, Vlad very nice video!! I have one question for you, you are using a periodic task for the FIFO instruction so, what time of execution do you recommend for this periodic task? Thanks!

[@benc8030]

Hi Vlad, Can you show an example of constructing product database that binds together with different attributes for i.e. barcode, height and etc using plc

[@fethilakhdari1078]

Thank you for a good presentation, I have tried the same logic on my "RSLogix Emulate 5000", although the behavior was somewhat similar, I noticed a few differences compared to what I can see in the video. Here are the most important: 1- The FFL Position increments from 0 to Length at each execution, whereas in your application the position remains at value 12 2- In my implementation the data is written downward i.e. from low array element index to high array element index (the opposite for your direction) I am puzzled as I did not see in your logic anything that explains the change in behavior, could you kindly provide any explanation for the observed differences in behavior? Thank you in advance for your help

[@antonioportugal9164]

How could I move the elements in an array? for example, I have 10 elements and I want to add a new one at position 0 by moving the one that was in 0 towards 1, 1 through 2, 2 through 3 and so on

[@benc8030]

I am using udt to do it and it would be beneficial if you can show me your way. I am a superfan of you.

[@davebennett5069]

FBD has a rolling average instruction already. Stand on the shoulders of those who came before you!

[@SolisPLC]

Every interview question has previously been solved. It's not about the "solution", but rather the path the candidate takes.

[@davebennett5069]

@@SolisPLC true, but I don't want prospective job candidates trying to build a can opener with their bare hands if a can opener is already on the table. Perhaps the path I take is not what this interviewer is looking for, but it is the path I take.

[@SolisPLC]

@@davebennett5069 again, it really depends on the goal of the interview. If the candidate mentions that the instruction is available in FBD, I can immediately tell that they're more than a beginner in PLC programming. The idea behind implementing a known function is that it's easy to explain the functionality and it's just as easy to validate the solution. The analogy you brought up is incorrect as "building a can opener" does not necessarily test your ability to use the can opener. In the case of a programming interview, writing logic that is implemented at the core of an advanced function tests your ability to use basic instructions to create a more complex one. This is very common practice in every single coding interview: in C++, you're always asked to build a class, you're used to build a data structure that exists, etc. You can always say "there is a library for this", but the reality is that your showcase of base programming skills can be easily tested. In either case, I'd be more than happy to hear what interview questions you've seen and would like for me to cover.

[@mrtrader4628]

WTF Hi Vlad way to fast for us new guys or just me... need to make it a bit more basic FIFO, load and unload Please