PLC / Analog Signals & Measurement Quality
Keep invalid analog quality separate from the last numeric value
A controller or HMI can continue displaying a last-known value after the measurement becomes invalid. Treating that number as fresh can mislead diagnosis.
Environment and inputs: Industrial PLC / automation guidance · Verify the exact controller, module, firmware, engineering tool and machine documentation. Linked vendor examples apply to their stated products and library versions; observation, state changes and physical work require distinct authorization.
Observe and record
Record raw channel status, data timestamp where available, PLC validity handling and HMI quality indication at the same observation time.
Interpret the evidence
A plausible number does not prove a healthy signal. Different platforms propagate stale, substitute and invalid values differently.
Resolve and verify
Implement the approved quality-handling design and clear operator indication through the controls owner, then validate documented loss and recovery cases.
Machine and data safeguards
Do not replace bad quality with a permanent good flag or a convenient constant to keep the screen green.