Set and Reset Priority: Simultaneous Boolean Requests Need a Defined Result

Retain the specific update rule

Beckhoff’s stated SR implementation is set-dominant: Qnew = S OR (Qold AND NOT R). Its corresponding reset-dominant RS rule gives reset priority. Primary reference: set-dominant implementation; reset-dominant implementation.

Use the actual documented block and parameter meanings. Two letters are not a sufficient migration specification across every vendor or instruction family. The comparison here concerns the explicit Boolean formulas, rather than presuming their names are universal.

An invented conflict trace

Take fictional old state false with S true and R true. The stated set-dominant rule yields true. The reset-dominant rule Qnew = (Qold OR S) AND NOT R yields false. If both requests then become false, each corresponding formula retains its previous result.

The conflict therefore changes the stored state beyond the instant when both requests were present. This example describes a generic notification flag and is not a machine-run, emergency-stop or safety-function design. No installed PLC program behavior is claimed.

Prepare a complete truth-table question

Record previous state, both requests, expected updated state and the exact block identity. Include the simultaneous case instead of testing only one request at a time.

Ask the application owner which priority the intended requirement defines. That review can identify a logic mismatch before a mnemonic is copied into a different library while preserving the actual initialization and execution rules as separate project evidence.

Customer Questions

Primary References

These references support the technical principles discussed in this guide. The worked examples and review questions are educational.

• Primary reference: set-dominant implementation
• reset-dominant implementation