Someone asked if TMR (or any Time based instruction) can be used inside a Task code block within the Do-more controller. The answer is a definite "that depends".
There are basically 3 primary ways a Task code block can be Enabled
1. Edge Triggered
2. Power-flow enabled with a NON-ZERO Time base (e.g. when ENTASK - Enable Task has power, run this task once every 500 milliseconds)
3. Power-flow enabled with a ZERO Time base (e.g. run it when ENTASK - Enable Task has power flow)
So in the example above, the Ladder View on the right is MyTask, which is a TASK code-block. It contains a simple 5.0 second TMR. The Ladder View on the left is $Main and it contains the 3 possible ways an ENTASK instruction can be used to Enable the logic inside a Task code-block.
1. Will run MyTask ONCE when X0 goes from OFF to ON. You will get one ladder scan's worth of time, which if your scan time is less than a millisecond, it will be 0ms
. This will not work.
2. Will run MyTask EVERY 500 milliseconds whenever X1 is ON. So every 500 milliseconds, you will get one ladder scan's worth of time, NOT the 500 milliseconds. This will not work.
3. Will run MyTask EVERY SCAN whenever X2 is ON. This means that X2 is a big Master Line Controller for the logic inside MyTask. THE TMR INSIDE MyTask WILL properly accumulate time. This is because the ENTASK is NOT edge triggered (it is power-flow enabled) and its Interval is 0ms, meaning that whenver the ENTASK has power, MyTask will run.
So X2 must be on for the X10/TMR logic inside MyTask to even execute. So if X2 is ON, then MyTask will run. But then X10 must be ON to enable the TMR. If X2 goes off (even if X10 is ON), TMR T0 in MyTask will reset because the X2/ENTASK rung in $Main indirectly controls the execution behavior in MyTask. Since there is no longer any power to the TMR instruction (whose input leg is an Enable/Reset type), then T0 will reset when X2 (or X10) goes OFF.
Oh, and if you actually TRIED to use a TMR instruction inside a Task code-block using method #1 or #2, the Program Check utility will flag #1 and #2, but not usage #3 (because TMR will properly accmulate time in #3).
(see the Check toolbar button in the online toolbar, it also gets run automatically when you download your project into your PLC)
Note that since PROGRAM code-blocks (when flagged to run) run every scan, so Program code-blocks do not have this issue like TASK code-blocks do.
Topic: TMRs inside Task code-blocks (Read 17447 times)