Structured Text Scan Cycle

damica1 · Feb 2, 2016

I think I understand how the scan cycle of a ladder program is done.

But if you create a section of code that uses Structured Text how would that get scanned?

Example:

for i := 1 to 5 by 1 do
AWT_1(TRUE, channel, data);
end_for;

Is each line a scan? How would this section excute?

Can someone explain this in more detail to me.

Thanks

nettogrisen · Feb 3, 2016

Structured text is scanned the same way that ladder logic is.

The FOR-loop in your example though, halts the scan untill i = 5 and then continues.
The same would be the case with a FLL instruction in ladder (in logix5k).

arkansascontrols · Feb 3, 2016

Structured Text [For Loops] halt the scan until the loop completes.

In cases where I can't tolerate the scan delay I usually build a multi-branch rung, use a counter that increments with each scan, and then on a parallel rung do your logic with an indirect address indexed off the counter.acc value. This will give you one step per scan but still functions like a loop.

Ron Beaufort · Feb 3, 2016

you might be interested in the "numerical" or "incremental" modes of the FAL (File Arithmetic and Logic) instruction ...

damica1 · Feb 3, 2016

OK, I'm still confused because of this. In my example:

for i := 1 to 5 by 1 do
AWT_1(TRUE, channel, data);
end_for;

Notice I now have an array, so AWT and most function won't excute(over and over) until the enable goes to false and back to true. So how do you make them work if they are in an array.

The loop above will not work because the AWT enable never goes back to false. What am I missing? Do I need to do something like this?

for i := 1 to 5 by 1 do
AWT_1(TRUE, channel, data);
AWT_1(False, channel, data;
end_for;

mk42 · Feb 3, 2016

damica1 said:
OK, I'm still confused because of this. In my example:

for i := 1 to 5 by 1 do
AWT_1(TRUE, channel, data);
end_for;

Notice I now have an array, so AWT and most function won't excute(over and over) until the enable goes to false and back to true. So how do you make them work if they are in an array.

The loop above will not work because the AWT enable never goes back to false. What am I missing? Do I need to do something like this?

for i := 1 to 5 by 1 do
AWT_1(TRUE, channel, data);
AWT_1(False, channel, data;
end_for;

I think what you would do is use a tag to that first parameter, where you currently use (TRUE/FALSE). When that loop executes each cycle, it will check the status of your trigger tag. I assume that is what you would do if you were calling an instruction like that in ladder.

There isn't anything magic about SCL or loops. It simply gives you an easier way to write what would have been large amounts of code. All the normal rules of scans still apply. You start at the top, and work your way down to the end. The next time the code is executed, you do it again. The FOR loop simply means you call that code 5 times, with "i" having a different value each time.

nehpets · Feb 3, 2016

I'm not too familiar with sending Ascii strings but shouldn't your command read AWT_1(channel,TRUE,data);

Can something like this be done (i don't have RSlogix 5000 in front of me right now)

for i := 1 to 5 by 1 do
AWT_1(TRUE, channel, data);
AWT_1.EN = FALSE;
end_for;

Steve

ndzied1 · Feb 3, 2016

According to the manual, the serial port instructions like AWT run asynchronously to the scan.

I would try to concatenate the data together and then send it out the serial port in a single AWT command.

damica1 · Feb 3, 2016

Just a reminder, I'm working with Connected Component Workbench.

This may work:
AWT_1.IN := False; "This would be equivalent to a ladder scan with AWT_1 "NOT ENABLED".

I will have to test.

MK42 I think I understand your concept here also, as it may work too!

Structured Text Scan Cycle

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