The invention relates to a control and/or data-transmission system comprising a number of I/O modules connected in series one adjacent to the other and a control and/or data-transmission module, as well as a control and/or data-transmission module for controlling I/O modules for such a control and data-transmission system. The invention proposes a control and data-transmission system that comprises a number of I/O modules connected in series one adjacent to each other, wherein each I/O module comprises at least one I/O signal channel and also at least one first signal terminal for connecting the I/O signal channel to a data bus and at least one second signal terminal for connecting a bus subscriber to the I/O signal channel, and wherein the system comprises a control and/or data-transmission module that comprises control electronics for the selective control of the number of I/O modules and that forms a detachable unit.