A modular system consisting of individual modules which can be mounted side by side in-line, comprising - a head module having at least one connection for an external bus signal on an external bus, at least one pneumatic supply connection, an electric supply connection, and: ▪ a serial bus interface for an internal serial bus, ▪ an electric supply interface, ▪ a multipole interface, and ▪ a pneumatic supply interface, which are each led out on the same side, - and at least one functional module having: ▪ an internal serial bus line, ▪ electric supply lines, ▪ electric multipole lines, and ▪ a pneumatic supply line, which are each passed through from one side to the opposite side and are each connected to a corresponding interface of the head module; - the head module converting serial bus signals into multipole signals and outputting these signals at the multipole interface; - the functional module selectively branching at least one of the multipole lines and, with a signal carried thereon, executing a pneumatic or an electric function or both a pneumatic and an electric function, wherein one expansion intermediate module is provided which can be inserted between two functional modules, which passes the internal serial bus line through from one side to the opposite side, which includes on one of its sides a multipole interface for the functional module attached thereto and which converts serial bus signals on the internal bus into multipole signals, characterized in that the expansion intermediate module has a separate electric supply connection for subdividing the modular system into segments which can be turned on and off separately, such that in case of an error, only the modules of one segment have to be disconnected from the voltage supply rather than the entire modular system, and in that the expansion intermediate module has a separate bus interface for feeding an external bus signal on an external bus, the bus interface being a diagnostic/programming interface, and the multipole lines comprising both incoming lines and outgoing lines, and the multipole lines in the expansion intermediate module being subdivided into multipole incoming lines and multipole outgoing lines. - a head module having at least one connection for an external bus signal on an external bus, at least one pneumatic supply connection, an electric supply connection, and: ▪ a serial bus interface for an internal serial bus, ▪ an electric supply interface, ▪ a multipole interface, and ▪ a pneumatic supply interface, which are each led out on the same side, ▪ a serial bus interface for an internal serial bus, ▪ an electric supply interface, ▪ a multipole interface, and ▪ a pneumatic supply interface, - and at least one functional module having: ▪ an internal serial bus line, ▪ electric supply lines, ▪ electric multipole lines, and ▪ a pneumatic supply line, which are each passed through from one side to the opposite side and are each connected to a corresponding interface of the head module; ▪ an internal serial bus line, ▪ electric supply lines, ▪ electric multipole lines, and ▪ a pneumatic supply line, - the head module converting serial bus signals into multipole signals and outputting these signals at the multipole interface; - the functional module selectively branching at least one of the multipole lines and, with a signal carried thereon, executing a pneumatic or an electric function or both a pneumatic and an electric function,