A modular programmable controller, comprised of a central processing unit and peripheral units, has a serial bus system in the form of a shift register. The total length of the shift register is equal to the sum of the individual register lengths of the bus interface connections of the peripheral units. The individual register lengths of the bus interface connections are dependent upon command and peripheral units. The individual register lengths amount to between 0 and 40 bits. This results in an exceptionally efficient exchange of information between the central processing unit and peripheral units.