Abstract: Electrical and electro-optical components are mounted on cards in various different sizes assembled to constitute working equipment using a set of common parts for building modular functional blocks of various sizes to house such cards. A modular functional block comprises one or more fixing bases each having a rectangular body having ends which engage respective retaining strips having grooves to form a printed circuit card-retaining frame. End plates are fitable to the various component parts mentioned above together with locking strips and closure cards to constitute a completed, enclosed functional block which can be built up to various different sizes.