Blocks for safe logic

Blocks for safe logic are used to develop safety-relevant applications .

For developing safety-related applications, you must use a logi.CAD 3 version that has been qualified for this purpose. See the documentation "Safety instructions on working with the IDE" for the appropriate information and valid safety instructions.

Base variant

The blocks for safe logic are particular system blocks that are highlighted as safe logic, if they are provided with a safe value as follows:

  • At least one block input (including the input EN) gets a safe value.
    A safe value is the value of a →variable declared based on a safe data type. Expressions according to ST (including operands in ST and operators in ST) can be used to evaluate safe values – as long as no variable is used that provides a non-safe value.
         Compare: A non-safe value is the value of a variable declared based on another →data type or an expression with such a variable.

  • For all other block inputs, one of the following conditions applies:

    • The input is unconnected, i.e. not connected to any other FBD-element.

    • The input gets a safe value (see above).

    • The input gets a constant value from a value field containing a →literal. Expressions according to ST (incl. operands in ST and operators in ST) can be used to evaluate the constant value – as long as no variable is used that provides a non-safe value.

Example: The system block OR is such a block for safe logic:
images/download/attachments/504365691/SafeLogicOR-version-1-modificationdate-1666090518100-api-v2.png
Explanation::

  • The left call is highlighted as safe logic because the 2nd function block input gets a safe value and all other inputs are not connected.

  • The right call is not highlighted as safe logic because the 1st function block input gets a non-safe value.

Special variant

The system block AND is a block for safe logic that is highlighted as safe logic, if it is provided with a safe value as follows:

  • At least one block input (but excluding the input EN) gets a safe value (see above).

  • For the input EN , one of the following conditions applies:

    • The input EN is unconnected, i.e. not connected to any other FBD-element.

    • The input EN gets a safe value (see above).

    • The input EN gets a constant value from a value field containing a literal. Expressions according to ST (incl. operands in ST and operators in ST) can be used to evaluate the constant value – as long as no variable is used that provides a non-safe value.

  • The other block inputs are not relevant for the identification of the safe logic. This means that these inputs can also get a non-safe value.

Here the behavior of the system block AND (based on the above example):
images/download/attachments/504365715/SafeLogicAND-version-1-modificationdate-1666090532762-api-v2.png
Explanation:

  • The left call is highlighted as safe logic because the 2nd function block input gets a safe value and the input EN is not connected.

  • The right call is also highlighted as safe logic. The same conditions as for the left call apply. Here the 1st block input is supplied with a non-safe value but this is not relevant for highlighting the safe logic in the case of the AND block.

List of blocks (base variant)

Be sure to use the documentation "Safety instructions on working with the IDE" to determine whether there are conditions for using the following blocks when developing safety-related applications, e.g. whether actions are required from the system integrator for the block to actually be supported.

The following blocks are provided as blocks for safe logic as of logi.CAD 3 version 3.20.0:

List of blocks (special variant)

The following blocks are provided as blocks for safe logic (special variant) as of logi.CAD 3 version 3.20.0: