Assign designated register blocks for specific PLC functions in order to validate data and block unauthorized access to controller data.

PLC best practices – Validate indirections by poisoning array ends to catch fence-post errors.

HMI access to PLC variables can be restricted to a valid operational value range but further cross-checks in the PLC should be added.

Alarm the operator when input/output states occur that are physically not feasible. Validate and alert for paired inputs/outputs in PLC.

If timers and counters values are written to the PLC program, they should be validated by the programmable logic controller.

Use cryptographic hashes, or checksums to check PLC program integrity and raise an alarm when they change.

One of the best PLC best practices is to use PLC flags as integrity checks. Put counters on PLC error flags to capture any math problems.

Leave as much operational logic e.g., totalizing or integrating, as possible directly in the PLC. The HMI does not get enough capacity.

Track PLC operating modes – Keep the PLC in RUN mode. If PLCs are not in RUN mode, there should be an alarm to the operators.

#1 PLC Best Practices – Split PLC code into modules, using different function blocks (sub-routines). Test program modules independently.