Before getting into how an addressable system works without an end of line resistor, or what is an end of line resistor (EOL), we need to look at what a fire alarm system is in the first place.
A Fire Alarm System (FAS) detects fire and warns people of the fire. It does this with detection devices (smoke and heat detectors, pull stations, waterflow switches, etc.) and with warning devices (fire horns, strobes, chimes, bells, etc.). Between the detection devices and the warning devices is a control panel and a large amount of building wiring.
The control panel does not detect fire or warn people of fire. According to alarms sent to it by the detection devices, the control panel only on the warning devices and controls a few other devices.
The wires also do not detect fire or warn people of fire. Connecting the detection devices and warning devices to the control panel, the wires only carry signals and power.
It is extremely important that all of the detection devices and all of the warning devices are able to send signals to or receive signals from the control panel. If a wire breaks or comes loose from a connection, some or all of the detection or warning devices won't work.
In other words, if a fire breaks out, unless a broken wire or loose connection is already fixed, the building could burn down and people could get hurt. or worse.
A secondary purpose of the fire alarm control panel is to constantly self-check the fire alarm communication system itself. The control panel supervises the whole system.
Addressable Fire Alarm Systems
To communicate between the control panel and the devices scattered throughout the building, the addressable fire alarm system communicates using computer style data. Also, to make sure devices are connected (supervision), the addressable fire alarm system uses the data to "Shake Hands".
In an addressable system, the control panel is looking for devices to send in their handshake signal. The control panel doesn't care about the wires.
Hand-shaking is an "I'm OK" signal. The control panel sends data down the Signaling Line Circuit (SLC) saying to a smoke detector "Hey! Smoke detector #27, are you OK?". Smoke detector27 then sends data back on the SLC saying "This is detector #27, I'm OK."
In order for the handshaking signal to get through from the device back to the panel, the wire (which is a communication path) has to be continuous and properly connected to both the device and the control panel. In other words, having received the handshake signal, the panel knows that the device is connected. The panel can then go on to check the next detection or warning device.
On the other hand, if the control panel doesn't receive this handshake signal from the device, the control panel sounds its trouble buzzer and turns on a warning light.
An addressable device is smart; an addressable device has circuitry that will handshake with the control panel.
Conventional Fire Alarm Systems
A conventional device is dumb; a conventional device does not have circuitry that will handshake with the control panel.
Basically, other than receiving an "Alarm" from a conventional device, a conventional control panel doesn't receive any signal from the device. If there is no fire, no conventional device will ever send any signal down the wires to the control panel.
Remember, if a device isn't connected (the wire broke or came loose from a connection) the fire alarm system either won't detect a fire or it won't warn people of the fire. Somehow, the control panel has to self-check (Supervise) to make sure the devices are connected.
Photo from the book "Make It Work - Conventional Fire Alarms"
In a conventional system, the devices can't send in a handshake signal, so the control panel has to check the wires to make sure devices are still connected.
A conventional Fire Alarm System (FAS) isn't capable of handshaking. To make sure the wires continue to carry signals and power, the only thing a Fire Alarm Control Panel (FACP) can do is check the wires to make sure they are connected.
It does this checking by putting an ohmmeter on the wires and making sure the whole length of the wire is continuous: no wire is broken or come loose from a connector or a device.
The ohmmeter, though, doesn't look like an ohmmeter. It looks like a zone input or zone output of the control panel. What the control panel does, though, is what an ohmmeter does. The control panel pushes electrical current through the wires to make sure the wires are con tinuous.
If anywhere along the wire the wire breaks or a connection comes loose, the current stops and the control panel turns on a buzzer and a warning light. In essence, the control panel is using its internal ohmmeter to check the continuity of the wires.
End of Line Resistor
Photo from the book "Make It Work - Conventional Fire Alarms"
The supervising current not just goes through the wires, but the current also goes through the terminals of the devices so if a device is removed, the current will stop.
In order for the ohmmeter inside the panel to check the wires to make sure they are continuous, there has to be a complete electrical path. Without an end of line resistor, the two wires in the loop do not conduct electricity between them; they show an open connection. To complete the electrical path through all the wires in the loop, the resistor is at the end of the circuit. (End of Line Resistor - Get It? Not everyone does.)
Essentially, being at the end of the line and not at the control panel, the end of line resistor requires the electricity to pass through all of the wires, so the whole length of wire is checked for continuity. In other words, to make sure they are complete and will carry fire alarm signals from the fire detection devices on through to the warning devices, the wires are being supervised by the control panel.
Handshake or End of Line Resistor
The preferred method of always checking to make sure everything is connected in a fire alarm system is to use a handshake, or if a handshake isn't possible, checking the continuity of the wire. The end of line resistor is only needed for checking continuity, it is not needed if there is a handshake.