A microprocessor-based unit to aid the teaching of digital systems design is described. The unit comprises a patchboard interfaced to a microcomputer which simulates logic blocks of any desired complexity; these may be physically interconnected on the patchboard to assemble a digital system. The configuration of logic blocks on the patchboard is specified by data in the microcomputer's memory; this may easily be changed to provide a range of systems experiments using the same equipment. The patchboard configuration may be described by a sequence of statements written in a special-purpose language, in order to provide a specification suitable for experimental procedure. The statements are processed by a translation program which generates the appropriate simulation code and data for the controlling microprocessor's memory.