A system of interlocking modules for use with a programmable logic controller. The system utilizes a plurality of modules, e.g. input/output modules, that are mechanically interlinked to obviate the need for a rack or back plane. Each module includes a movable plug portion that selectively may be moved into engagement with a corresponding plug receptacle of the next adjacent module. The mechanical interlocking features and the plug portions are designed to permit insertion and removal of individual modules disposed between adjacent modules, without moving either of the adjacent modules.