A method of determining a configuration state of an I/O module connected to a fieldbus controller is disclosed, the method comprising the steps of receiving from the I/O module a key ID, comparing the key ID with at least one key ID stored on the fieldbus controller, and configuring the I/O module when the received key ID does not correspond to the at least one stored key ID stored. Further, a circuit for configuring a digital input of a fieldbus controller as either a PNP input or an NPN input is provided comprising a control circuit including a reference input, a first selection input, and a control output. The circuit also comprises a data circuit including a second selection input electrically connected to the control output, a data input electrically coupled to the digital input, and a data output.