A plurality of system control devices respectively include network interfaces for communicating with each other via a network. One of the plurality of system control devices functions as a master system control device, and the remaining system control devices function as slave system control devices. The master system control device transmits a control command to the slave system control devices by means of the network interface. The slave system control devices receive the control command by means of the network interfaces and supply an operation command to abatement devices in accordance with the control command. When unable to communicate with the master system control device, the slave system control devices either function as master system control devices or enter stand-alone operation mode.

A bus system is provided. The bus system includes a master device, a bus, and a plurality of slave devices electrically connected to the master device via the bus. 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. When a first slave device communicates with the master device through the bus, in a first phase of a plurality of phases in each assignment period, the first slave device sets the alert-handshake control line to a first voltage level via the alert handshake pin, wherein the first phase corresponds to the first slave device. In the phases other than the first phase in each assignment period, the alert-handshake control line is at a second voltage level. Each of the phases includes two clock cycles.

A bus system is provided. The bus system includes a master device, a bus, and a plurality of slave devices electrically connected to the master device via the bus. 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. When a first slave device communicates with the master device through the bus, in a first phase of a plurality of phases in each assignment period, the first slave device sets the alert-handshake control line to a first voltage level via the alert handshake pin, wherein the first phase corresponds to the first slave device. In the phases other than the first phase in each assignment period, the alert-handshake control line is at a second voltage level. Each of the phases includes two clock cycles.

A plurality of system control devices respectively include network interfaces for communicating with each other via a network. One of the plurality of system control devices functions as a master system control device, and the remaining system control devices function as slave system control devices. The master system control device transmits a control command to the slave system control devices by means of the network interface. The slave system control devices receive the control command by means of the network interfaces and supply an operation command to abatement devices in accordance with the control command. When unable to communicate with the master system control device, the slave system control devices either function as master system control devices or enter stand-alone operation mode.