Realize half-duplex conversion from RS-232 interface to RS-485 interface. When applying RS-485 network communication, because the whole communication system is connected through RS-485 bus and each control unit is far from each other, to ensure the system reliable, RS-485 interface terminal adopts the separate driving mode in each channel and photoelectric isolation with 3500V isolating voltage to connect host-linked RS-232 with RS-485 interface terminal. The high-speed transient voltage suppresser (TVS) can effectively restrain lightening and ESD, and effectively avoid lightening strike or other interferences.