The following steps should be taken if your TDC4 displays "tOPN" even with the probe connected.
Verifying the continuity of your RTD probe:
Connect a multimeter between the two pins shown in the figure below, using the notched part of the connector for reference. A resistance between 100 and 150 ohms indicates the probe has proper conductivity. This does not confirm the accuracy of the probe--only it's operation.
Verifying the continuity of the TDC4 RTD receptacle:
Use a small length of wire and a resistor (100-150ohms) to connect the pins as shown in the figure below, using the keyed part of the receptacle for reference. When the TDC4 is switched on it should read a small, negative value of temperature rather than the "tOPN" warning. This does not confirm the accuracy of the TDC4 tempeture reading--only proper operation of the RTD receptacle.