Disclosed herein is a slave module for monitoring an electric system. The slave module includes: a data transmitting/receiving unit configured to receive request data from the master module and transmit response data to the master module via the serial bus; a data converting unit configured to convert the request data into first digital data and convert second digital data into the request data; a determining unit configured to determine whether the serial bus is in inactive state based on the first digital data and the second digital data; and a data processing unit configured to transmit emergency data to the master module if it is determined that the serial bus is in the inactive state.