Synchronizing Redundant Data

When the second PLC is started, the redundancy data (with AREA_INPUT redundancy area type) is copied first and one time from the active PLC to the second PLC. Then the PLC switches to standby.

However, you can deselect the default AREA_INPUT redundancy area type to exclude variables from the synchronization.

When the configuration file of the runtime system (*.cfg) contains the following entry, a synchronization is triggered every time a task cycle is started.

[CmpRedundancy]
DataSyncAlways=1

The variables of PROGRAM objects and global variable lists whose redundancy area type is AREA_INPUT are affected.

PROGRAM PLC_PRG
VAR
    b: INT;
END_VAR

IF sReduState.eRedundancyState = RDCY.STATE.RS_CYCLE_ACTIVE THEN
    b := b + 1;
END_IF

If the configuration file of the runtime system (<rts name>.cfg) does not contain the entry DataSyncAlways=1, it may be preferable to trigger a synchronization programmatically in the IEC code.

To do this, call the RedundancySynchronizeData() function in your IEC code. Then the variables with the AREA_INPUT redundancy area type are synchronized. Therefore, check the settings on the Registered Area tab on the Variables tab. The copy operation is performed at the beginning of the task cycle.

IF bUpdateData THEN
    bUpdateData := FALSE;
    RedundancySynchronizeData();
END_IF

The AREA_SYNCHRO redundancy data of type can be transferred during operation in each task cycle from the active PLC to the standby PLC. The size of the data is limited for this. The assignment of the type is not supported in CODESYS, but has to be done programmatically as a callback of the AreaRegister function.

The area has to be registered before the application starts. It is too late to register areas in the first cycle. That is why the registration is done in the callback of the cbDownloadDone system event.

Example 10. Example

FUNCTION cbDownloadDone : DWORD
VAR_IN_OUT
        EventPrm: CmpApp.EVTPARAM_CmpApp;
END_VAR

(* Register GVL_Synchro to be synchronized in every task cycle *)
bAreaRegisterDone := AreaRegister(ADR(iSyncFirst), ADR(iSyncLast) - ADR(iSyncFirst) + sizeof(iSyncLast), AREA_TYPE.AREA_SYNCHRO);
tNowDownloadDone := TIME();

The call of cbDownloadDone is performed when the OnlineChangeDone system event occurs. System events are added to the task configuration.

The AREA_PASSIVE redundancy data of type can be transferred during operation in each task cycle from the standby PLC to the active PLC. The size of the data is limited for this. The assignment of the type is not supported in CODESYS, but has to be done programmatically as a callback of the AreaRegister function.

The area has to be registered before the application starts. It is too late to register areas in the first cycle. As a result, the registration is done in the callback of the cbDownloadDone system event (in the same way as in the example from the previous chapter).

Example 11. Example

FUNCTION cbDownloadDone : DWORD
VAR_IN_OUT
    EventPrm: CmpApp.EVTPARAM_CmpApp;
END_VAR

(* Register GVL_Synchro to be synchronized in every task cycle *)
bAreaRegisterDone := AreaRegister(ADR(iSyncFirst), ADR(iSyncLast) - ADR(iSyncFirst) + sizeof(iSyncLast), AREA_TYPE.AREA_PASSIVE);
tNowDownloadDone := TIME();

The call of cbDownloadDone is performed when the OnlineChangeDone system event occurs. System events are added to the task configuration.

The value of the IEC time base is transferred from the active PLC to the standby PLC in every PLC cycle. This affects TIME, TON, and TOF. The system times (example: SysTimeGetMs) are not transferred and are available as a local time on each PLC.