Modular computer system and I/O module

Without being restrained to a specific bus scheme, kinds of I/O modules connected to a processing module can be discriminated. Module exclusive selection parts respectively provided in I/O modules connected in a stacked form to a processing module via connectors judge only a module select signal input from terminals in the same position on processing module side connectors to be active. Based thereon, identification information of its own I/O module is output to a predetermined terminal on the connector. Without being restrained to a specific bus scheme, therefore, the processing module can acquire identification information of the I/O modules from a predetermined terminal on a connector. One I/O module can be selected by a simple module selection circuit scheme of inputting module select signals successively output from the processing module to terminals in the same position on processing module side connectors according to the connection order of the I/O modules.