A modular bus system, comprising: - a support rail having a base surface and a longitudinal axis; - at least one base module mountable to the support rail for electrical connection to at least one input/output functional module (31 - 36), which has a backplane circuit board with at least one set of at least two first electrical connectors (70a, 70b) connected thereto, for electrical connection to an electrical connector of an input/output functional module (31 - 36), wherein the at least two first electrical connectors (70a, 70b) of the at least one set are arranged one after the other along an imaginary line that extends perpendicular to the longitudinal direction of the support rail, and lie in the same plane as the base surface of the support rail or in a plane parallel to said base surface; - at least one input/output functional module comprising an electrical connector and an electronic device for communication with the modular bus system, wherein the electrical connector of the input/output functional module (31 - 36) is configured to be electrically connected to one of the electrical connectors (70a, 70b) of the at least one base module; wherein an electronic device is arranged on the backplane circuit board of the at least one base module, which is configured to communicate with the electronic device of the at least one input/output functional module (31 - 36); wherein on at least one side of the backplane circuit board of the at least one base module, which extends perpendicular to the longitudinal axis of the support rail, a second electrical connector is provided for being electrically coupled to a complementary second electrical connector of a further base module, wherein said complementary second electrical connector is arranged on a side of a backplane circuit board of the further base module, which extends perpendicular to the longitudinal axis of the support rail; characterised in that the electronic device of the at least one base module is electrically connected to the first electrical connectors (70a, 70b) and to the at least one second electrical connector and includes an evaluation and control device that is configured to detect whether an input/output functional module (31 - 36) is connected to any one of the first electrical connectors (70a, 70b), and if so, to which one or more of the first electrical connectors (70a, 70b) an input/output functional module (31 - 36) is connected to. - a support rail having a base surface and a longitudinal axis; - at least one base module mountable to the support rail for electrical connection to at least one input/output functional module (31 - 36), which has a backplane circuit board with at least one set of at least two first electrical connectors (70a, 70b) connected thereto, for electrical connection to an electrical connector of an input/output functional module (31 - 36), wherein the at least two first electrical connectors (70a, 70b) of the at least one set are arranged one after the other along an imaginary line that extends perpendicular to the longitudinal direction of the support rail, and lie in the same plane as the base surface of the support rail or in a plane parallel to said base surface; - at least one input/output functional module comprising an electrical connector and an electronic device for communication with the modular bus system, wherein the electrical connector of the input/output functional module (31 - 36) is configured to be electrically connected to one of the electrical connectors (70a, 70b) of the at least one base module; wherein