> For the complete documentation index, see [llms.txt](https://infosys.selmotech.com/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://infosys.selmotech.com/selmo-solution/selmo-solution-en/selmo-how-it-works/selmo-training/chapter-3-the-advanced-standard-functions-in-selmo/3.2-paircheck-contradiction-check-as-a-safety-principle.md).

# 3.2 PairCheck – contradiction check as a safety principle

#### Idea

A cylinder can physically only be in one place at a time.\
Either front or rear.\
But if both end-position sensors report a signal at the same time, there is a contradiction – and thus potentially a dangerous fault.

Selmo detects this case automatically through the **PairCheck**.\
The PairCheck is a logic integrated into every zone that checks whether two or more feedback signals exclude each other.

#### Procedure

1. Cylinder 1 has two feedbacks: *Front* and *Rear.*
2. In the zone description it is specified that these signals belong together – a so-called *PairCheck group.*
3. During runtime the system continuously compares their states.
4. If both are TRUE or both are FALSE, the PairCheck detects the logical error.
5. The result:
   * The zone automatically sets an interlock.
   * The automatic release is lost.
   * A red diagnostic appears in the HMI:\
     *“PairCheck Error – Contradictory feedback.”*

#### Benefit

This completely eliminates a common source of error.\
It is impossible for a cylinder to be “seen” in two positions at the same time or to fail unnoticed.\
The PairCheck is therefore an **integrated safety monitor**, not an additional module.

<figure><img src="/files/1cd9b6855d4047fb5638c14e4e47be85bd9e7256" alt=""><figcaption></figcaption></figure>


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