PLC Tutorials

#11 PLC Best Practices – Instrument for Plausibility Checks

Instrument the process in a way that allows for plausibility checks by cross-checking different measurements.

Security Objective	Target Group
The integrity of I/O values	Product Supplier Integration / Maintenance Service Provider

Instrument for Plausibility Checks

There are different ways of using physical plausibility for validating measurements:

a) Compare integrated and time-independent measurements

Plausibility checks can be done by integrating or differentiating time-dependent values over a period of time and comparing to time-independent measurements.

b) Compare different measurement sources

Also, measuring the same phenomenon in different ways can be a good plausibility check.

Different measurement sources do not necessarily have to be different physical sensors, but can also mean using alternative communication channels (see examples).

Example

a) Compare integrated and time-independent measurements

Metered pump and tank level gauge: volumetric change should equal integrated flow.

The burner in a boiler: added caloric heat should equal temperature rise.

b) Compare different measurement sources

Using air-speed, artificial horizon, vertical speed, and altitude in the airplane to measure the phenomenon of the climbing / descending airplane.

Comparing process parameter values from independent data loggers (tied into 4-20mA loops or relay contacts and transmitted via independent communication channels) to SCADA system data (coming in the “normal” way through PLC and HMI) and alerting on deviations and significantly off-specified values.

Why?

Beneficial for…?	Why?
Security	Facilitates monitoring for manipulated values (assuming not all sensors are manipulated at once).
Reliability	Prevents acceptance or identifies (for future action) corrupted / wrong measurements as inputs.
Maintenance	Rules out the possible physical causes for failures more quickly.

References

Standard/framework	Mapping
MITRE ATT&CK for ICS	Tactic: TA010 – Impair Process Control Technique: T0806 – Brute Force I/O
ISA 62443-3-3	SR 3.5: Input Validation SR 3.6: Deterministic Output
ISA 62443-4-2	CR 3.5: Input Validation CR 3.6: Deterministic Output
MITRE CWE	CWE-754: Improper Check for Unusual or Exceptional Conditions