Forced Function of Variables for Emergency How to Use the Forced Function of Variables for Emergency Production?

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 How to Use the Forced Function of Variables for Emergency Production?

Today, in this article, we want to share a useful feature for equipment maintenance and emergency production: the Forced Function of PLC variables.




What is the Forced Function of PLC variables? We know that the value of variables in PLC depends on the program's execution, and different conditions can lead to different outcomes for the variable. For example, consider the simple code below:



Under normal circumstances, the output result of Q0.0 depends on the conditions of I0.0 (Start) and I0.1 (Stop). When I0.0 = 1 and I0.1 = 0, the output result of Q0.0 is 1. However, there is a way to keep the value of Q0.0 always at 1 without modifying the program code. This method is known as the Forced Function of variables.

The Forced Function of variables can be used for equipment maintenance and emergency production situations. For example, if a proximity switch (or other sensor) fails, causing a motor to be unable to start, and there are no spare proximity switches available temporarily but the motor needs to start urgently, the Forced Function can be used to force the corresponding output point of the motor, ensuring uninterrupted production.

Now, let's take the example of CPU 317-2PN/DP to explain how to use the Forced Function:

In the project tree of TIA Portal V13, locate "Watch and force tables." Expand it to find the default "Force table," as shown in the image below:


Note:

  1. Each CPU has only one "Force table," and it cannot be added or deleted.

  2. To use the "Force table," the programming computer (PG/PC) must be connected to the CPU (Go online).

Double-click on the "Force table," and on the right side, you can add the variables you want to force, as shown in the image below:


In the "Force Value" section, enter the value you want to force (TRUE or FALSE), click on the "Start Force" icon, and a dialog box like the one below will appear:


Click "Yes" in the dialog box, and all variables listed in the table will be forced with the "Force value" instead of the program-calculated values.

Forced variables are a temporary measure. Once the fault is corrected, the forced function for the respective variables needs to be canceled. How to do it?

Similarly, in the "Force table," click the "Stop forcing" button, as shown in the image below:

A confirmation dialog box will appear:


Click "Yes" to cancel the forced function for variables (all variables in the table).

Regarding the forced function of variables, here are some points to note:

  1. PLCs in the S7-300 series (except for 318/319) can force input points (I) and output points (Q).

  2. PLCs in the S7-400 series (including 318/319) can force not only input points (I) and output points (Q) but also M points and peripheral points (PI & PQ).

  3. When the forced function is enabled on a CPU, the "FRCE" light on its front panel will be illuminated.

  4. Power cycling the CPU does not cancel the forced variables. To cancel the forced function, you must select "Stop forcing" in the forced variables table.

The forced function of variables is suitable for emergency production situations. After the fault is eliminated, it is recommended to cancel the forced function. Additionally, because the forced function ignores the program's calculation results, it may lead to the inactivation of certain safety conditions. Please use it with caution.