RSLogix 500 Ladder Logic Please Help!

[Mark Snodgrass]

I want to clear up what Seth said, so you understand what synchronous and asynchronous mean.

The PLC5, Slc500 Family processors cycles in a certain order:

1. Scans the input conditions and writes that data to the data table.
2. Scans the program left to right, top down until it comes to end of main routine (it will jump out for subroutines, but will return to the same spot).
3. Reads the data table and turns outputs on or off based on the data table.
4. Performs PLC overhead and housekeeping
5. Return to 1.

The RS5000 Family of processors cycles differently which has an effect of speeding up the overall scan time.

The program scans right to left, top down same as the PLC5s but the inputs are updated by the input module at a scheduled interval. The inputs can change anytime within the execution of the logic. The output modules read the data at a scheduled interval.

What this does is if you use an input early in the logic, it may change before the logic uses it near the end of the logic. This can give unexpected results to the logic. Also outputs may not change when you expect them to. This can be a problem with large slow programs and/or fats processes. What the 'mapping' does is to at the beginning of the program we set internal bits with the status of the inputs, so they stay the same throughout the entire scan. Then at the end of the program the outputs are 'mapped' so that all the logic has been completed before changing what the physical outputs do.


Look at Ron Beaufort's videos that I gave you the link to and it explains it a bit clearer than I can.

 

[seth350]

That is a lot clearer Mark, but I believe you had a small typo..
"The program scans left to right, top down same as the PLC5s.."

 

[GlenGineer]

There are days where I do the same task I have done a hundred times before and then there are days when I learn and do new things. I thought the main difference between RSlogic 500 and 5000 was tags. I learned several things new today...thank you.

 

[Bit_Bucket_07]

That is a lot clearer Mark, but I believe you had a small typo..
"The program scans left to right, top down same as the PLC5s.."

Yep... Still, the concept is correct. The program logic isn't scanned or solved differently in ControlLogix processors. The IO updates are simply asynchronous with the program scan.

This is also true of reads and writes from HMIs with the older PLC and SLC processors. If the HMI references are not buffered, then one section of your program logic can see a different state for HMI writes than the rest of the program logic sees and the HMI will not always see the last state of data that is read from the PLC if that data is modified in multiple instances within your program logic. You are unlikely to observe this phenomenon with a very small test application because the PLC program will solve so quickly, but it is critical to be aware of this asynchronicity when developing a larger application.

 

[jaywillnot]

Hey Seth,
Sorry for the slow response time...very long weekend. Your answer to the mapping issue I was seeing is exactly as you explained. I had forgotten that you could map field inputs to tags in 5000. I now recall a friend at work who is an AB guru, told me how to look for unnamed inputs, etc, by looking up the tag by way of the "map." Does that sound close?

Thank you for the link. I'll open it up in just a bit. Kind of busy tonight lol

 

[jaywillnot]

Wow, awesome responses guys! I feel way out of my league here, but it only makes for a healthy challenge to better myself...

thank you all for your support. You guys rock \m/