Mixing Safety-related and Standard Types in SNOLD
This topic contains information on the following:
| Note
Term definition: Standard = non-safety-related The term "standard" always refers to non-safety-related items/objects. Examples: a standard process data item is only read/written by a non-safety-related I/O device, i.e., a standard device. Standard variables/functions/FBs are non-safety-related data. The term "standard controller" designates the non-safety-related controller. |
Why mixing safety-related and standard variables?
In SNOLD networks safety-related and standard variables can be mixed.
This way, the enable principle can be programmed without using the EN_OUT function, as described in the topic "Programming the Enable Principle".
The easiest way of realizing the enable principle in an SNOLD network is to program a logical AND connection of a safety-related signal (inserted as LD contact) with a standard signal (also an LD contact). This means that the standard controller (represented by the standard signal in the code network) agrees to the Safety PLC (represented as safety-related signal). This is shown as first example network below.
In such mixed SNOLD networks, AND connections of safety-related and standard variables as well as type conversions from safety-related to standard data types are allowed. Data type conversions from standard to safety-related as well as wired ORs of safety-related and standard data types writing a safety-related variable are invalid. Refer to the rules described below.
Safety rules
When mixing safety-related and standard variables, the following safety rules must be observed:
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WARNING
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Unintended machine operation
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Safety-related signal paths emphasized
When mixing safety-related and standard variables, the SNOLD editor performs a data flow analysis in the code and highlights the leading safety-related signal paths of a network by displaying them as yellow shaded lines.
A safety-related path always ends either at a safety-related output variable or, in case of a standard output variable, at the last object input located before this output. If a standard signal path ends at a safety-related output, this output is shown with a red marking. Additionally, an error message appears in the Error List ("Standard input writes on safety output" in the example below).
Examples:
Treatment of formal parameters
In code networks, FB/function inputs are treated like output variables of the network. Conversely, each output formal parameter of an FB/function is considered like an input variable of the network. As a consequence, the same rules which are valid for variables apply to FBs/functions when connected in a mixed network.
Example:
Admissible connections of safety-related and standard variables
The following connections between safety-related and standard variables are allowed:
Safety-related input variable writes standard output variable (corresponds to type conversion safety-related > standard).
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Safety-related and standard input variable write safety-related output variable (AND connection).
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Safety-related and standard input variable write standard output variable (AND connection).
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Two (or more) parallel safety-related input variables write standard output variable (corresponds to wired OR with following type conversion safety-related > standard).
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Safety-related input variable and parallel standard input variable write standard output variable.
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Safety-related input variable writes safety-related output variable with parallel standard output variable.
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Inadmissible connections of safety-related and standard variables
The following networks contain invalid connections between safety-related and standard variables resulting in a compiler error message:
Standard input variable writes safety-related output variable (corresponds to invalid type conversion standard > safety-related).
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Safety-related input variable with parallel standard input variable write safety-related output variable (wired-OR of safety-related and standard data type with following invalid type conversion).
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