Standard Tags in Safety Routines (Tag Mapping)
A safety routine cannot directly access standard tags. To allow standard tag data to be used within safety routines and synchronize standard and safety actions, safety controllers provide a safety tag-mapping feature that copies standard tag values into safety task memory.
Mapped tags are copied from the standard tags to their corresponding safety tags at the beginning of the safety task. The copying process can increase the safety task scan time.
TIP:
Standard task routines can directly read safety tags.
Because a download is required to change tag mapping, mapping a structure of information provides programming standardization and flexibility during commissioning.
Mapped Tag Structure Example
The following example shows how to qualify standard data with safety data.
Qualify Standard Data with Safety Data
Item | Description |
|---|---|
A | Latch circuit to help prevent automatic restart if the standard input (MappedTag) is failed in a ‘stuck at 1’ state. |
B | Safety input qualifier for mapped tag |
C | Safety output |
Restrictions
Safety tag mapping has these restrictions:
- The safety tag and standard tag pair must be controller-scoped.
- The data types of the safety and standard tag pair must match.
- Alias tags are not allowed.
- Mapping must take place at the whole tag level. For example, myTimer.pre is not allowed if myTimer is a TIMER tag.
- A mapping pair is one standard tag that is mapped to one safety tag.
- You cannot map a standard tag to a safety tag that has been designated as a constant.
- Tag mapping cannot be modified when any of the following are true:
- The project is safety-locked.
- A safety signature exists.
- The controller switch is in RUN position.
- A nonrecoverable safety fault exists.
ATTENTION:
If you use standard data in a safety routine, you must verify that the data is used in an appropriate manner. Standard data in a safety tag is not safety data. Do not directly control a SIL 2 or SIL 3 safety function by using standard data.
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