In the earlier post, we saw how to write a for-do statement. It was used to perform a task within certain criteria and if the criteria was finished the execution was finished. The same task is performed by a while do statement, but in a different way. While a certain condition is true, the statements will be executed. Now you may think that it is similar to a for do statement, but there is a slight difference. Let us see in this post how to write a while-do statement in SCL language.
What is a while-do statement?
In the for-do statement, you had to specify two limits within which the condition will be checked and the statements will be executed. But in the while-do statements, two limits are not required. A single condition is checked, and if this condition is satisfying, then the statements will be executed in a continuous loop. When the condition goes false, it goes out of the loop and exits. So the general meaning is that – while the condition is true, execute the statements in a loop and when the condition goes false, come out of the loop.
The general syntax for this construction is –
While Condition Do
Statement;
End_for;
The meaning is – check for the condition, execute the statement and come out of the loop. Then again check the condition, execute the statement and come out of the loop. This loop will continue until the condition goes false. Once false, the loop is exited and the PLC executes other logic written.
If it is required to exit the loop in between, then mention a keyword named exit; in between. Due to this, the loop terminates without executing fully. You can also write this under an if-else condition, to come out forcefully from the loop. One thing to remember is to not execute a loop under an already existing loop; otherwise it will become infinite and the PLC will go into fault mode.
How to write a while-do statement?
Refer to the below image. The logic is – while the value of seconds is more than 0, decrement it’s value every 1 second by 1. The initial value of this tag will be 10. Once this value becomes less than 0, then the decrement will be stopped. For your understanding, I will also use the exit keyword to understand more better.
The execution flow is as follows –
- We have fixed or initialised the value of the secs as 10.
- First, the secs is initiated to 10, so that abrupt addition does not execute.
- So, as this value is more than 0, it jumps inside the loop where a timer instruction is first written for executing every 1 second. After every 1 second, we decrement the value by itself to 1, as shown in the 47th line.
- The loop is then exited through exit keyword. Then, it comes out of the overall construct and again goes back to the condition. As the value has now become 9 and still more than 0, the loop is executed once again. This goes on till the value becomes 1. The loop is executed one last time and when it becomes 0, the execution stops and the loop is exited finally.
You could have written this statement in an if-else style too. Simply replace the while statement as – if secs>0 then, remove the exit keyword and replace end_while with end_if. But, this is good for our example of simple decrement. If complex statements are required to be executed continuously in a loop, then you would have to write an if-else statement, which would make the program bulkier. In that case, a while loop statement makes a task much easier to work.
In the next post, we will see how to make use of comments in SCL language.
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