Recent security alarm systems use EOL(R) or DEOL(R) to terminate the line of the zones.
Question 1: What method is used to measure the resistance of a zone?
I can think of two options:
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It measures voltage drop on the zone using a current source
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The zone is part of a voltage divider, voltage measured on the zone
Question 2: How good/bad idea is to use a simple voltage divider for that?
UPDATE
Since not too much answer arrived, I have done some experiment.
I bought 100m 6×0.22 shielded cable and tried out the voltage divider option. I had different scenarios, I tried to put noise on the line by coiling the cable on a 2000W electric heater's input line and switching it on and off. I used 5V and 0.9mA to drive the line. I was watching the line with an oscilloscope but did not notice anything. I did not even use a single capacitor as a filter, I used only the shielding. Without the shield, of course, there were very ugly peaks when I switched off the heater. Although, I think if I would have used a current source, that would also be affected by such an EMI.
Since there is no load on the voltage divider, there is only a very minimal current which allows longer cable without significant voltage drop. Despite of that, I don't think that is common to use a cable for a zone which is longer than 100m.
There were concerns about installation contractors, who don't care about specifications, noise supression etc.. These people are out of scope this question. If someone decides to use something improperly then he has to deal with the consequences. In our country the installer can be punished if the alarm system doesn't work correctly and the house get robbed because of that.
However, I'm not a specialist of this field, I just want answers, that's why I test and suppose.
Best Answer
My alarm uses a simple voltage divider. It has a 5 V power bus. Each sensor input is pulled up with about 4k7. The common return of all sensors is wired to ground.
In normal end-of-line resistor mode, the input node will be held at about 2.5 V by the two resistors. A sufficient deviation from this voltage means the zone has gone open or short circuit.
I suppose the alarm simply uses a many-channel A/D, or a multiplexer, to sample the zones. There is some protection and conditioning of each input to guard against RFI and ESD I suppose.
My alarm goes one step further and allows "zone doubling" where two sensors with eol resistors can be connected. One uses about 1k, the other 6k8. You can see that these two will generate a unique voltage for (almost) any fault condition.