Abstract: A coding system for various I/O module elements of an I/O cabinet assembly is disclosed which is able to identify analog, digital and control I/O modules by appropriate functional representation combined with a distinct color code found on the front of each module and to identify appropriate internal field wiring connections in the identified module by the module identifying color. Each module has a written identification such as A—for analog, D—for digital and C—for control, I—for input, O—for output combined with an appropriate color bar identifying the type of module. Thus process and terminal blocks forming and I/O unit are easily matched for assembly and are easily inserted into an I/O cabinet in the proper location.