A bus system is provided. The bus system includes a master device, an enhanced serial peripheral interface (eSPI) bus, a plurality of slave devices electrically connected to the master device via the eSPI bus, and a first resistor. Each slave device has an alert handshake pin. The alert handshake pins of the slave devices are electrically connected together via an alert handshake control line. The first resistor is coupled between the alert handshake control line and a power supply. Each slave device obtains the number of slave devices according to a first voltage of the alert handshake control line.