A modular peripheral unit, which monitoring a technical process, includes a base module to which at least one expansion module can be attached in series, the modules each having a module upper part and a module lower part, as well as having at least one profile permitting serial attachments. The module lower parts have a multiplicity of essentially U-shaped contact elements in the region of the profile with which they can be attached in series. One limb of the contact elements extends into the interior of the module lower part, such that the multiplicity of these limbs form an internal male contact strip and the multiplicity of remaining limbs of the contact elements form an external male contact strip. When an expansion module is attached in series, the external male contact strip engages in the expansion module such that the external male contact strip is opposite the internal male contact strip of the expansion module which is attached in series. The desired connection between the internal and external male contact strips can be achieved within the module with suitable measures.