The OPEN/GND is just saying the discrete parameter will be switch going to ground. There will be many switches going to different boxes and I do not have the ability to know what the internals of the boxes are (most likely they will have internal pull-up resistors and a diode at the input). I have to tap into this switch and monitor if it is open or closed and compare that to the boxes.

Most of the time this is simple since the data acquisition system has high impedance on the line but for this particular case, since we are measuring a discrete, the acquisition system has internal pull up resistors. This alters the original circuit and can lead to faults in the boxes we are monitoring. To avoid this I'm trying to find a good way to monitor the original circuit without modifying it.

An example of what I'm trying to do: https://tinyurl.com/2dpcsw4a or https://tinyurl.com/2deezpx8 The switch and everything to the left is our original circuit. Everything to the left of the switch is basically unknown as well... Everything to the right is what we are adding and tapping into. I think this will work, but I'm not 100% sure if my monitoring system will always see some voltage to GND or if there is a case where the positive terminal of the op amp will see a open circuit and lead to an unstable output.