Unit double board connection circuit

There is provided a switching circuit for unit duplication boards designated as a working board and a protection board. In each duplication board, a software block is maintained reset at an initial hardware operation and determines the activation or deactivation state of a board according to a software command received from a central processing unit (CPU), a hardware block monitors a clock signal received from the other board using a system clock signal for the board and determines a time point when the system clock is to be activated according to the monitoring result, and a hardware board state block determines the working or protection state of the board in hardware terms, reflecting the states of the board and the other board during hardware operation. One of the unit duplication boards is automatically set to a working mode at an initial hardware operation, for hardware switching.