Terminal resistance automatic coupling method and system of programmable logic controller (PLC) expansion bus

The present invention discloses a terminal resistance automatic coupling method of a PLC expansion bus. The method is applied to a PLC control system which is formed by concatenating a PLC and a plurality of expansion modules via the expansion bus and possesses a bus topology structure, and comprises the steps of setting the in-place signal lines in a forward bus interface and a backward bus interface of each expansion module respectively, wherein the in-place signal lines possess the in-place signals of default levels; when the power-up initialization is carried out on the PLC, using each expansion module to generate an in-place signal of a specific level, and transmitting the signal to the in-place signal line of the backward bus interface of the prior level expansion module via the forward bus interface of the current-level expansion module; using the expansion module to determine whether the current-level expansion module is the last expansion module in the bus topology structure according to the level type of the in-place signal of the backward bus interface; if yes, matching the terminal resistance of the current-level expansion module to the expansion bus. The present invention also discloses a corresponding system. By implementing the technical scheme of the present invention, the terminal resistance can be matched automatically and rapidly.