Don,
The End Of Line [or Loop] Resistor AKA EOL Resistor is what loads the zone's loop circuit in normal operation.
The typical EOL resistor would be 2K ohm and connected to TTL logic gates. The circuit's operating voltage is like 5 VDC, so the current through the loop is 2.5 milliamps [0.0025 amps].
What this does is sets up a constant flow of current at a certain level. If a sensor trips and closes the N.O. contacts inside of it, which are wired in Parallel for each device, this causes the loop current to rise above the 2.5 Ma level and activates the alarm.
On the same note, if someone tampers with a sensor and causes the N.C. contacts which are wired in series with all devices to open, this will cause the loop current to stop flowing and will also activate the alarm. Same will happen if the loop's wires are cut.
If you need more info, let me know!! I am back from the Honeymoon and ready to write again!!
Scott. "S.E.T."
