The application program is where a correct specification can still become a wrong function. The process sector expects the program to be developed with the same discipline as the hardware.
Practices
- Use the logic solver's limited variability language (function block diagrams, ladder) and its certified function blocks as stated in the safety manual.
- Keep the structure simple: one function per section, inputs, voting, logic and outputs clearly separated.
- Use naming that matches the C&E and the SRS.
- Avoid using the safety PLC for non-safety control.
- Review the program against the SRS and C&E by someone who did not write it.
- Test every row of the C&E, including degraded voting, time delays, overrides and resets, in factory and site acceptance tests.
- Version control: record the version and checksum; the proof test checks they match the approved version.
- Change control: every change through management of change, with impact analysis and retest.
In the SRS
First Out AI has an "Application program" field written into the SRS; the proof test draft includes the program version check.
Written for First Out AI as engineering guidance. It supports engineering work and does not replace it; check every value against your standards, certificates and company rules with a competent functional safety engineer.