A field device I/O connector includes a base and an I/O module removably mounted on the base for transmitting I/O signals along an I/O channel. A set of electrical connectors mounted on the base connect the base to a field device. The electrical connectors, the base, and the I/O module mounted on the base cooperate to define the I/O channel. The I/O channel includes a conversion circuit to convert between I/O signals and a digital data format output from the field device I/O connector. The base and I/O module also cooperate to define a diagnostic circuit connected to the portion of the I/O channel extending through the I/O module. The diagnostic circuit extends to a diagnostic logic circuit that may be included on the base, in the I/O module, or away from the field device I/O connector.