Douglas Krantz - Technical Writer - Describing How It Works
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Why Isn't an End of Line Resistor Used the Same Way as a Terminating Resistor?

The End of Line Resistor completes a circuit; like a continuity tester, the control panel uses the resistor to check the continuity of wires. A Terminating Resistor absorbs signals; like a power absorber, the terminating resistor prevents the signal from bouncing back from the end of the circuit.

The Terminating Resistor is in the same exact place in the circuit as the End-of-Line Resistor. But there has to be a difference.
Used in signal transmission (data, radio frequency, etc.), the termination resistor is at the end of the wire circuit.

Used in fire alarm and security systems, the end-of-line resistor is at the end of the wire circuit.

What's the difference?

By Douglas Krantz

In electronics, at the end of the communication line, there's often a Terminating Resistor. In fire alarm and security systems, at the end of the input circuit (Initiating Device 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 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.
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