Drilog logging while drilling system is developed by CNOOC Oilfield Services Co., Ltd. (COSL) during the 12th Five Year Plan period. Due to the special high temperature and high pressure environment and the special connection mode between the instruments, a single-core bus is needed for the interconnection between the instruments. The single-core bus is used not only for power supply, but also for signal transmission. The mechanical drill collar shell of the instrument is connected as the system ground to realize the power supply and signal multiplexing. This paper focuses on the influence of the key device parameters of the single core tbus (tool bus) bus on the bus carrying capacity. It analyzes and studies the design and selection method of the key device parameters. A physical interface of power supply and signal coupling is designed. Through field experiments and 300 well tests, the results show that the physical interface design can effectively reduce the waveform distortion of the bus signal and improve the load capacity of the bus system.