Hi all!

I have been in the alarm business for over 38 years. Most of the systems I deal with utilize an end of line resistor for supervision of the circuit and it also allows the use of open and closed circuit switches on the same circuit. With this in mind, intrusion circuits are usually closed loop devices and fire circuits use both types of devices with specifically open circuit devices activating the fire loop and closed loop devices use for supervision of the fire circuit. An example of this type of fire circuit is a 4-wire smoke detector. At the end of the loop is an end of line relay circuit that monitors the power going to the smoke detector with the end of line resistor after the smoke detector's open contacts so that even if the end of line resistor was missing, you can still generate an alarm on that loop.

Anyhow, these types of circuits are usually fed into an LM339 voltage comparator so that both "open" and "shorted" loops can be detected. I use this type of design in things that I make and it works great. The loop has a resistor (the industry uses 1K, 2.2k, 4.7k and sometimes 5.6k values in their designs). There are one or two companies that even use 3.3k. There are some companies that utilize the A-D or comparator inputs on micros to do the analysing for them saving having to put another IC on the board. The input circuit looks like the second circuit shown in the example without the TVs and usually only have a .1uF capacitor in the circuit.

Hope this information helps you with your design!

BobK