> 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-structure-and-standard/selmo-technical-standard-and-structure/description-selmo-plc-code/sequence-logic.md).

# Sequence logic

The sequence logic refers to the actual control and diagnostic logic for the step sequences. The PLC segment layout specifies how the sequences are arranged one after another. Each step chain logic is divided into four different parts.

1. **Standard Beginning** The "Standard Beginning" determines the operating mode of the step chain logic and the start conditions; it monitors the error status of the sequence to allow automatic or manual operation.
2. **Sequence Logic Control** The "Sequence Logic Control" area provides the logic that is responsible for the actual control and monitoring of each individual machine movement/action.
3. **Sequence Constantly Monitored Zone** The "Constantly Monitored Zone" or C.M.Z. provides the logic that constantly monitors dangerous machine states such as the failure of fuses, circuit breakers, etc. Any such fault leads to an immediate stop of the sequence.
4. **Standard End** The "Standard End" section contains logic that monitors the step state to control the step counter and format error information that is sent to the HMI diagnostic software package "Selmo HMI".


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