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Looking for a known resistance on a cable run

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Going back to the OP's original post.

1. Let's assume the line is unpowered.

There are easy to work with constant current sources that I can think of. 1) One from Linear technology and 2) One from TI. Add a blocking diode, just in case you run into some DC.

Then use a window comparitor to give you the indications you need. a) Above b) Below and c) right on.
Line length is a big variable.

Here's the TI (LM334) part: https://www.ti.com/product/LM334

Here's the LT part (LT3092): https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=4&cad=rja&uact=8&ved=0ahUKEwiHyIjHzdTKAhUCQj4KHa6oAqUQFgg1MAM&url=https://www.linear.com/product/LT3092&usg=AFQjCNFJ7mg1SlKE9r1oeqRSGZe_gNxYpQ&sig2=UWu5hAHOVUMRPmLAeXOSkg

A window comparator and reference (LT6700) is here: https://www.linear.com/product/LT6700

You might have modes of:
a) Window test (push button)
b) DC presence (push button)
c) On/Off switch
d) A current selector

(b) would say have a comparator looking for typical alarm voltages.
e.g. 12 and 24 with hysteresis. e.g. >1V, withing 10% of 12, within 10% of 24.

(a) would apply an applicable current around 10 mA.

Depending on the termination resistors that you want to look at, you can decide if currents or changing the voltages would work. Voltages might be harder. 10 mA is a good number because it breaks down some of the oxides.

Devices of this sort always have battery life issues without the ability to turn off when unused for a length of time.
A "battery test" function might be desirable.

You could make a 12 and 24 V version.

The "FULL DESIGN" would have to know the expected terminators, tolerences and hysteresis.

While, it might be cool to say connect the desired termminator and have the circuit work from there.
You could have a "variable" one two.

I have to "throw stuff out" for consideration without knowing the details.

Some simple ideas are 12/24 VDC and check for DC presence.
Allow a fixed number of different terminator values selected by a switch.
Have some means of creating a custom one.
Have some means of selecting the wire length: e.g. 50, 100, 1000 ft.
Have some means of selecting the wire AWG.

These are just being thrown out.

Aside: I found an WIFI voltmeter on youtube, so you could be fiddling out in the building somewhere and measuring the voltage at the panel with your cell phone.

Another aside: When I was servicing power amplifiers, I found it convenient to make "adapters" for the speaker connections. e.g.
1) alligator clips
2) Banana plugs
3) Braid

to my standard connector to the load/scope.
 
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I have had both good and bad luck with cable tracers. If only the white wire is broken but the black wire is good the cable tracer will not work well.

I don't know if the 24 volts is AC or DC.
If you break in to a cable and look back at the monitoring machine you should see 24V and I don't think the exact voltage is important. So here is a simple 1 resistor 2 LED tester. The RED LED will light or the GREEN LED will light or both if the voltage is AC.
The monitor machine will see about 5k.
upload_2016-1-31_18-3-3.png

Looking at the resistor side:
Two 9V batteries. Two LEDs and 2 resistors.
If the wire is open one LED will light.
If the wire is shorted the other LED will light.
If the resistor is some thing near the value of R3 no LED will light.
A small error will make one LED glow very dim.
 
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