Abstract: A method and a device for half duplex serial signal control with multiple baud rates and multiple configurations comprises RS-232 interface circuit, RS-485 interface circuit, power circuit, protective circuit, and logical control circuit. A brand new controlling method is applied to the receiving/transmission signal control wire on the RS-485 interface circuit so that the programmable logic controllers (PLCs) in the two-wire RS-485 network can be set at different baud rates and different configurations for RS-232 communication. That is, programmable logic controllers (PLCs) from different manufacturers, using different communication speed can connect one another as well as a personal computer. A signal converter transforms the RS-232 interface signal to the RS-485 interface signal for each PLC, passes through the RS-485 signal in the two-wire RS-485 network, and finally transforms the RS-485 signal to RS-232 signal for the connected computer.