I apologize for the confusion and appreciate your willingness to try and help me despite my lack of clarity.
First, to respond to your understanding of what I said:
A) Yes, I do have a system I am monitoring. It is a rather complex control panel.
B) System M does have nodes that are either 24VDC or 0. The system I am monitoring has it's own PLC in it. The nodes I am monitoring are the outputs of that PLC.
C). Yes, I am using a PLC ("P") that are internally pulled up to 24VDC.
D). Yes, you are understanding correctly. As I wired it up I found that whether or not the system "M" is outputting 24VDC or 0VDC the plc "P" inputs read 24VDC. This is, I believe, because both of them are pulled up and there is no path to ground.
E). There is a typo in the drawing. That is intended to be 24VDC. It is the power supply used to power the PLC "P" as well as the system "M".
F). The transistor is my best guess at what I can use to deal with the problem that should up in D. It provides a method to pull down the "P" input and uses the node from "M" that really I want to sense as the Base for the transistor.
G&H). You are correct. That relay is part of system "M" and is activated or not by a different mess of events. In essence, that is what I am attempting to detect is either open or closed.
1)The connections at the bottom of the PLC are the power connection into the PLC (V+ and GND).
2). The relay is controlled by "M" "has a set of it's own inputs that it is sensing, performing logic upon and then opening or closing the relay.
3). The 2VDC should be 24VDC and it powers pretty much everything, the PLC "P", the system "M", and is the power passing through the relay.
4). I cannot put the relay in place between the P input and ground because the relay is embedded in system "M". The only reason I showed the relay was because I didn't know if I needed a diode across it (to protect the transistor) and/or if I need a resistor between the power supply and the base of the transistor.
5). Are they sharing ground? Well, some of the inputs are. Others are not. However, I could change things a little and make them share the same ground if it helps.
As for your final comment, I did not build the system with a transistor at all. However, I did not realize that the PLC input was internally pulled up (sourcing), I thought it was a "sink" or internal pulled down. The issue I have is that I cannot change the system "M" whose nodes can only be monitored by a "sinking" input channel in P. Since I am stuck with internal pulled up inputs in P, I am looking to build a little circuit that can deal with this. My ide was using a transistor as a switch, but need to know if i require any resistors, diodes, etc to prevent causing some sort of damage or failure.