With the release of V7.20, a feature called persistence can be applied to I/O devices. When implemented, the tag cache for an I/O device is written to an XML file at a set period, allowing the last available data to be reloaded following a shutdown.
Persistence is enabled using the Persist field in the extended section of the I/O Devices Properties dialog.
You can also create a persisted memory I/O device using the Express Communications Wizard. When this type of device is selected, the wizard creates a new I/O device with the Address and Port Name properties left blank, Memory set to “TRUE”, and Persist set to “TRUE”. The File Name field is left empty to allow runtime to generate a default cache file name. This file will be located in the [DATA] directory.
Migrating Disk I/O devices
When applied to I/O Devices in memory mode, persistence provides an improved alternative to a disk I/O device, as synchronization is supported in scenarios where a server becomes unavailable for a period of time.
Many customers use disk I/O devices to provide system-wide global variable tags that are managed by I/O servers and are persisted to disk to maintain their latest values. Disk I/O devices take advantage of the standard I/O system redundancy features, such that, if one I/O server is unavailable, another can provide client(s) with tag values. They also perform a level of synchronization by using features such as standby write and by providing redundant paths to the persisted binary data files, so that, at startup of an I/O server, the latest value can be read into the system from the latest modified data file.
However, there is no synchronization when network connections are inoperative and regained, resulting in several scenarios in which redundant disk I/O devices can end up with different values for the same tag. For this reason, it is recommended that data assigned to disk I/O devices be migrated to the new persisted memory I/O mode available in V7.20.
To migrate disk PLC devices to persisted I/O memory mode: