Big picture question on instrumentaion logic. My existing setup is a single level transmitter (4-20mA) feeding circuit via a controller. To avoid a failure of the circuit, we would like to add a second level transmitter (so that one LT fails, the other will continue to work and feed the controller and its circuit). However, the controller circuit is too complicated, we would like to do the following:

Take 2 LT inputs (2 x 4-20mA), run it through a black box. Out from the black box, we need a single 4-20mA signal for the controller and the downstream circuit. The black box should be also able to know a failed LT (fail low or fail high or power supply failure) and switch to the working transmitter automatically.

Is such black box device possible? Is anything commercially (readily?) available? If so, please provide the detail.

I appreciate all inputs and responses.

(PS: Controller failure is also similar to the LT failure; but at this point we want to make LT fail proof and worry about controller later)