Abstract: Plug-in circuit modules and a prewired panel are provided for constructing fully functional relay-type control circuits according to ladder diagram conventions. Plug-in modules include input converters for interfacing external switch devices to the relay equivalent logic circuitry, output converter modules for interfacing the logic circuitry to external load devices, relay contact equivalent modules, relay coil equivalent modules, and wire equivalent modules which can be used to effect certain circuit interconnections. The panel is prewired to interconnect module plug-in positions such that insertion of modules produces continuous electrical circuits across the panel much like a ladder diagram. Contact equivalent modules can be addressed to effectively link them to selected coil equivalent modules to establish a master-slave relationship.