Two-wire fluid temperature transmitter with thermocouple diagnostics

A two-wire process fluid temperature transmitter comprising:

• a power module couplable to a two-wire process control loop to solely power the transmitter with power received from the two-wire process control loop;
• a loop communicator coupled to the power module and couplable to the two-wire process control loop, the loop communicator configured to communicate over the two-wire process control loop;
• a thermocouple input couplable to a thermocouple to receive a thermocouple signal;
• measurement circuitry coupled to the power module and the thermocouple input, the measurement circuitry adapted to measure voltage across the thermocouple input;
• a current source coupled to the power module and the thermocouple input, the current source adapted to selectively pass a diagnostic current through the thermocouple input in a diagnostic state, and pass substantially no current through the thermocouple input in a normal state;
• a controller coupled to the power module, the loop communicator, the measurement circuitry, and the current source, the controller adapted to receive a measurement signal from the measurement circuitry during the normal state, and a diagnostic signal from the measurement circuitry during the diagnostic state, the diagnostic signal related to resistance across the thermocouple input, the controller adapted to provide an output to the loop communicator related to at least one of the diagnostic signal and measurement signal;
• wherein the current source is adapted to pass the diagnostic current in alternate directions during the diagnostic state;
• and
• the diagnostic signal relates to voltages resulting from the diagnostic current flow in the opposite directions.