A motherboard includes a main body and two relay devices attached on the main body. The main body includes a signal output terminal, a signal input terminal, a first connector module, and a second connector module. Each relay device includes a circuit board, a relay, and a third connector module. The input terminal outputs an input signal to the corresponding relay via the first and the third connector modules in turn. The relay corresponding to the signal input terminal outputs the input signal to the signal output terminal via the third connector module corresponding to the signal input terminal, the first connector module, the second connector module and the third connector module corresponding to the signal output terminal in turn.