How to configure PID controller on a thermocouple failure
Voted best answer
I'm not sure what do You mean by "configure the PID controller to fail", but in case of sensor failure (thermocouple in Your case) the most basic solution is to set the controller in some kind of safe mode. That can be achieved if You have any kind of failure signal from sensor (like the mentioned open circuit signal). You can assign it to variable and connect to track input TRC of Controller Function Block. With this and safe output value connected to OTA and OTM inputs the controller will set the safe value as an output on failure detection, ignoring for the time being calculated output value.
It is not a perfect solution though - first of all You have to be sure of Your failure signal - if it would give You false positive even for a short duration, it will be a big disturbance to Your controlled process. Secondly, you should put a switch or something similar on TRC input to regain control over proces in case of need.
It's only one of possible solutions. You should choose one that is appropriate to Your process.