Abstract: The invention concerns I/O modules for a data bus, each of which consists of a base contact block, which can be set upon an assembly rail. Each base contact block has a flat bottom channel, in which the data bus lines and the current supply for the I/O electronics units on the circuit boards are guided, whereby I/O electronic modules for each connection plane or for individual groups of connection planes of the base contact block can be plugged into printed circuit boards arranged in the bottom channel, whereby a simple handling and a high modularity is given.