Field bus coupler for connecting input/output modules to a field bus, and method of operation for a field bus coupler

A method and apparatus for operating a field bus coupler in a normal operating mode to connect at least one input/output module with the control computer of an industrial automation system, including receiving from the control computer via a field bus a control output value that is transmitted via a sub-bus to the at least one input/output module, and receiving from the at least one input/output module via a sub-bus a control input value that is transmitted via the field bus to the control computer. During a diagnostic mode, the apparatus is operable to assign to the at least one input/output module via the sub-bus at least one desired diagnostic output value, and to assign to the control computer via the field bus at least one desired input value.