5.10. Continuity Check

The continuity check feature is implemented using the gc_Extension( ) function and the associated GCEV_EXTENSION event.

The structure associated with the GCEV_EXTENSION event (METAEVENT structure) contains the extevtdatap field which is a pointer to an EXTENSIONEVTBLK structure. The value of the ext_id field in the EXTENSIONEVTBLK structure can be:

• S7_EXT_CONTCHECK to indicate the beginning of a continuity check process
• S7_EXT_CONTCHECK_END to signal the end of a continuity check process

The parmblk field in the EXTENSIONEVTBLK structure contains additional information. The parmblk field, which is of type GC_PARM_BLK, contains only one element of parameter data of type GC_PARM_DATA. The set ID of this parameter is S7SET_CONTCHECK and the parameter ID is S7PARM_CONTCHECK_TYPE. The parm_data_size is sizeof(int).

In this feature, the gc_Extension( ) function does not require any GC_PARM_BLK data, except when sending continuity check result and the outcome of the test must be sent. Also, the gc_Extension( ) function does not return anything via the retblkp parameter.

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