Specification of the Safety Function
You must create a specification for your safety function. Use this specification to verify that program logic correctly and fully addresses the functional and safety control requirements of your application. In some applications, the specification can be presented in various formats. However, the specification must be a detailed description that includes the following (if applicable):
- Sequence of operations
- Flow and timing diagrams
- Sequence charts
- Program description
- Program printout
- Written descriptions of the steps with step conditions and actuators to be controlled, which includes the following:
- Input definitions
- Output definitions
- I/O wiring diagrams and references
- Theory of operation
- Matrix or table of stepped conditions and the actuators to be controlled, including the sequence and timing diagrams
- Definition of marginal conditions, for example, operating modes and emergency stop
The I/O portion of the specification must contain the analysis of field circuits:
- Sensors (digital or analog)
- Signal in standard operation (dormant current principle for digital sensors, sensors OFF means no signal)
- Determination of redundancies that are required for SIL levels
- Discrepancy monitoring and visualization, including your diagnostic logic
- Actuators
- Position and activation in standard operation (normally ON)
- Safe reaction/positioning when switching OFF or power failure
- Discrepancy monitoring and visualization, including your diagnostic logic
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