EOL Resistor not a Termination Resistor

Early security systems used simple electrical circuits to monitor the status of doors and windows. The circuit was either closed or open, and therefore returned full voltage or no voltage at all to the control panel; that was all the system wanted or needed to know. Although such circuits are still in use today, the digital age gave manufacturers the opportunity to make systems more secure.

A resistor is a small semiconductor which resists the flow of electrical current. The current is permitted to flow, but is reduced by the value of the resistor. If a resistor is connected, in series, with a sensor on an alarm circuit, then the control panel no longer sees full voltage across the circuit, but rather a reduced voltage, when the circuit is closed. Now there are three possible conditions for the control to measure: full open-circuit voltage (if the circuit is open), reduced voltage (if the circuit is closed and secure), and no voltage if the wiring has been compromised. For if the two sides of the circuit are making contact at some point between the control and the resistor, the current has a shortcut back to the control, thus bypassing the resistor. The control will see this no voltage (or very low voltage) as a fault and will alert the user.

In fire alarm and security systems, at the end of the input circuit (Initiating Line Circuit or IDC), and at the end of the output circuit (Notification Appliance Circuit or NAC), there's an End-of-Line Resistor. 

Both the Terminating Resistor and the End-of-Line Resistor (EOL) are across the conductors at the end of the circuit. 

The difference between the two types of resistor, though, is not where they're at or even what they're made out of. They're both at the end of the line, and both across the conductors. They can even come out of the same package from the store or distributor. 

The difference between the two is why they're installed at the end of the line. 

Terminating Resistor

In signal carrying systems, especially between equipment like between the TV camera and the video recorder (Closed Circuit TV or CCTV), or an RF amplifier for a Master Antenna TV (MATV) system and the TV set, there's a Terminating Resistor. 

On a Video Input, inside the DVR (Digital Video Recorder or Security Recorder) there's a Terminating Resistor. 

With Master Antenna TV (MATV), the Terminating Resistor is at the last "tap" when several taps are used in a single line from the amplifier. 

When the TV set is at the end of the line for the MATV system, inside the TV set there's a Terminating Resistor. 

For a true RS485 communication control circuit, there's Terminating Resistors at each end of the daisy-chain. 

Purpose of the Terminating Resistor

A Terminating Resistor is a Signal Quality component. It is used to soak up an AC signal, preventing reflections or ghosts on the line. 

It takes time for signals to travel along wire, and the energy of the signal cannot be created or destroyed; it has to go somewhere. At the end of the line where the wires end, if the wire is just cut off, the signal hits the end of the line and bounces straight back. 

If the wires at the end are shorted together to prevent the bouncing straight back, the signal takes a U-turn and returns in reverse voltage. 

Because of the time it takes for the signal to travel to the end of the wire and back, the bounced back signal is an extra, unwanted signal on the line. It degrades the video or control signals, or if the wire is long enough even returns as an extra erroneous video or control signal. 

The value of the Terminating Resistor is chosen very carefully to be able to soak up this signal when it reaches the end of the line so the signal doesn't bounce back. 

End-OF-Line Resistor

The End-of-Line Resistor used in fire alarm systems and security systems may look the same as a Terminating Resistor, however the function of the End-of-Line Resistor is completely different. 

The "End-of-Line Resistor" confirms that the wiring in a building remains undamaged. It's across the end of all the wire in each pair of wires (loop) for all inputs (Initiating Device Circuit or IDC) and outputs (Notification Appliance Circuit or NAC). 

Purpose of the End-of-Line Resistor

As opposed to the terminating resistor, which is there to prevent signal reflections, the End-of-Line Resistor is a Life-Safety and Property Protection component. 

It's important to understand that the "signals" used in fire alarm systems are DC, either on or off, not AC, which carries information like video or data. 

Because there's no AC, there are no reflections from the end of the loop; the End-of-Line Resistor is used to pass DC current. 

The only purpose is to pass a small supervision current so the fire alarm or security panel can "look" at the wire; if the supervision current stops, the stoppage can be reported immediately, and the trouble fixed, before there is a real fire. 

The End-of-Line Resistor versus the Terminating Resistor

So the Terminating Resistor of RS485 control wiring or coax cable is used to soak up AC signals, while the End-of-Line Resistor of the fire alarm or security system loop is used to pass DC current.