def scan_chain_len(self, scan_chain):
        """Retrieves and returns the number of bits in the scan chain.

        Args:
          self (JLink): the ``JLink`` instance
          scan_chain (int): scan chain to be measured

        Returns:
          Number of bits in the specified scan chain.

        Raises:
          JLinkException: on error.
        """
        res = self._dll.JLINKARM_MeasureSCLen(scan_chain)
        if res < 0:
            raise errors.JLinkException(res)
        return res