Here’s a Main program with just a single rung of ladder code.
Let’s create a task, Call it Shift Report, and let’s tell it to NEVER yield.
In that Task, we’ll create a For Loop that increments D0 from 1 to whatever is in D100.
Normally we would log data or something like that inside this loop, but that’s a subject
for a different video.
So for our example we’ll just use a math instruction with some trig functions just
to kill time.
Back in Main, we’ll enable the task and make it edge triggered so it only runs once,
which is usually what you would do, because normally you would trigger an end of shift
report on some event or during a certain time each day.
But for this demo, we’ll add a control bit so we can force it whenever we want to.
Great, so we have a main scan loop that has one rung of ladder code and one rung that
enables our task when C0 is set.
Accept that.
Write it out to the PLC.
Put the PLC in run mode.
Let’s create a dataview with our control bit, the loop counter and the value the loop
counts to.
Let’s make that 250 loop iterations.
Let’s also monitor the actual scan time… the average scan time… and the max scan
time.
These numbers are all in microseconds.
Looks like it takes on average this many microseconds to execute the program without the task enabled.
If we enable the task - holy cow – during the one scan that ran the task it maxed our
scan time out at 40 times the average scan time!
Why is that?
Well, because we told it to NEVER yield.
On every scan we’re executing this entire loop.
I we increase the number of loops, the scan time goes up.
Again, we are running through the entire loop on each scan – that is, we are never yielding.
Let’s re-configure the task to Always Yield, and write that back out to the PLC …
Let’s clear that max scan time number.
With the Task disabled we get this scan time, and when we enable the task it only bumps
up the scan time just a little bit.
Why?
Because since we setup the Task to Always Yield, so the PLC does one pass through the
loop and exits the task.
On the next scan it does another pass through the loop.
Next Scan, another pass.
Etc.
So we are only executing one iteration of the loop per scan.
What if we change the loop count to 1000?
Look, no change in the max scan time!
Why?
Well, since we told the task to only do one pass of the loop on each scan it doesn’t
matter how many passes through the loop we need!
Let’s take a look at the last Yielding option.
Let’s tell the task it can take up to 1000 microseconds but then it has to yield.
Write it out to the PLC.
With the task enabled; our scan time maxes out at a little over 1000 microseconds.
Exactly what we expect.
We call this time we allotted to the task a time slice.
And look at this.
You can change the time slice while the task or program is running.
It’s just one of the structure members of the program or the task.
If I change the time slice, and re-execute the task, then sure enough, the scan time
changes accordingly.
Can you use Yielding with a Program?
Sure – look it’s the exact same thing.
Just like a Task, whenever a Program hits a Yielding instruction, it reacts however
you setup Yielding for that Program.
So, once again Do-more gives YOU total control over how your PLC operates.
That’s so cool.
One Caution when using NEVER Yield: There is a Watchdog in the PLC.
If your code takes too long to execute, then it’s going to assume something is wrong
and it’s going to reset your PLC.
Or, it may shutdown depending on how you have your watchdog setup.
We’ll cover the Watchdog in another video.
Just be careful your code doesn’t exceed that watchdog time limit when using never
yield or when setting up time slices.
That ought to give you a pretty good feel for how all of this fits together.
The main things to remember are:
Tasks are great for executing things that only require a one pass through the code – even
it takes multiple scans.
Programs can do everything tasks can do but they can also do anything that might require
multiple passes through the code where you have to keep coming back to check on something
like Stage or Asynchronous Communications.
And finally, Yielding is an awesome way to reduce your scan time.
If you need any help with Do-more Tasks and Programs, please contact AutomationDirect’s
free, award winning support team during regular business hours.
They will be happy to help.
And don’t forget the forums!
There are lots of automation professionals there that love to share their years of experience.
Just don’t post any questions directed at AutomationDirect’s support team there, they
don’t monitor the forums on a regular basis.