Persistent Storage

OpenPLC can be configured to use persistent storage. Persistent storage periodically writes certain PLC addresses to disk. When the runtime starts, OpenPLC initializes these PLC addresses to the last written value prior to executing the first cycle.

Persistent storage can be used to state information locally.

Persistent storage stores all values that are marked as “memory storage”, that is, with the M storage class. Other storage classes (namely I and Q) are not stored.

Important

When enabled, persistent storage periodically writes data to disk. Hardware that utilizes a flash-based drive have limited write cycles; writing to persistent storage will reduce the life-time of the drive. If your application requires persistent storage, you may need to choose an appropriate write interval based on trade-offs between freshness of data and write cycles.

Configuring Persistent Storage

Persistent storage is disabled by default. You can enable persistent storage either via:

web-based configuration
config.ini configuration file in the pstorage section

Changing Programs

Important

This section describes capabilities only in the development branch. You must checkout the development branch to use these capabilities.

The persistent storage file contains a signature derived from the list of PLC addresses.

Changing the running program may change the signature and prevent the application from loading the persistent storage.