typedef struct MEINodeRealTimeBit {
    union {
        long	*Src;	/* Host address */
        long	*Dst;	/* Host address */
    } Data;
    long	BitMask;
} MEINodeRealTimeBit;

The SERCOS RealTimeBit can be in the Status Word or Control Word. This is why Data can be either a Src or Dst. These pointers point to a 32-bit word. Since we are only looking at a bit, BitMask is used to mask off all the bits that are not of interest.