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. 

Working of Fire Alarm Wiring

Working of Fire Alarm Wiring

Fire alarm systems save lives and protect property. Fire alarm systems also break down because they're electrical.

During fire, if a wire breaks, Class A Wiring provides an alternate route for signals to pass between field devices and the fire alarm panel.

A Signaling Line Circuit (SLC) carries information in the form of data to and from the field devices for the fire alarm system, and also carries power from the control panel to the devices.

Class A or Class B wiring loops/zone help the fire alarm panel to find and fix these breakdowns (faults) before a fire, while there is time for repairs.


Class B Loops

Normal Class B wiring - All devices are supervised and working

In conventional / non-addressable Class B Loops / Zone Wiring, all devices are daisy-chained together. By watching a small electrical current passing through the wires, the panel supervises them, and to limit this supervising current, at the end of the daisy-chain is an end-of-line resistor. The panel constantly watches for this current. 

The current, as it leaves the panel, goes out one of a pair of wires, goes through a current limiting resistor called an End-of-Line resistor (EOL), and returns on the other wire. This pair, along with the EOL, makes up a fire alarm loop. Devices are connected to this loop so their connections, also, receive the supervision current. 

Open Fault in the Class B wiring. Supervision tells the panel that the wiring does not go through, but also the devices further from the panel don't work.

If the supervising current stops flowing, the panel assumes a wire is broken (an open fault), and displays a trouble. When a wire breaks in Class B, the devices closest to the panel will still work, but because of the wire break, the devices further from the panel are cut off. Class B sometime called Style 4 loop.

Class A Loops

Normal Class A wiring - All devices are supervised and working.

Under normal conditions, Class A Loops  sometimes called Style 7 loop are similar to Class B Loops, but with an important difference. 

Class A wiring takes error detection further than Class B. If a wire breaks, the panel uses a redundant wire path to maintain communication with devices beyond the break. Here even though a wire is broken, all devices work.

To keep more devices working, Class A uses a second path from the fire alarm panel; a redundant wire loop goes around the broken wire. A fire can still be detected, because, using this redundant path, most, if not all, devices on the loop remain connected to the panel. So loop makes addressable type, now the word came SLC.

Basically, when the fire alarm panel detects an open wire in the Class A Loop, it automatically switches to using two separate un-supervised Class B loops. The first one is the original Class A loop, and second one back-feeds on the separate pair of wires to make the second Class B loop.

Most of the devices on the original Class A loop will be on either the first or the second Class B loop.

Separation on Class A Wiring Routes

The question is asked: In Class A wiring, how close to the feed wires are we allowed to get the return wires?

The answer is: It depends. During a fire, as the fire damages the wiring, how badly does one want the fire alarm system to continue to work?

1.      The feed and return wires can be in the same bundle or conduit if there's no concern.

2.      The feed will be in one part of the building and the return will be in the opposite part of the building if one is concerned about life safety.

True Class A wiring schemes make sure to protect the redundant return path by routing it through the building on a separate route.

The concern here is that whatever breaks a wire in the first part of the loop might break all the wires in the same bundle. An example: A forklift tears through all the wires in a bundle at once. If both feed and return wiring routes use the same wire bundle, and the whole bundle of wires is broken, and all the devices beyond the break will not communicate with the panel.

In that case, Class A wiring will not be any better than Class B.

The NFPA Code does allow for some exceptions, but mostly the code says the outgoing wiring path and the incoming wiring path should be separated by some distance.

Resetting Class A Troubles

Most fire alarm panels automatically restore trouble messages when the trouble is repaired. However, because the Class A Loop isn't supervised the same way as Class B Loops, the fire alarm panel can't detect corrections. 

With Class A faults, after correcting the open fault, resetting the panel will clear the trouble message.

Bottom Line for Class A

Class A Loop wiring uses both a primary wire path, and a redundant secondary wire path.

When a wire breaks, by using both paths, devices are still able to communicate with the fire panel. 

Install End of Line Resistance

Install End of Line Resistance

End of line resistors (EOLR) are resistors of a specified value that are used to terminate protective loops or zones.

The purpose of EOLR's is to allow the control panel to supervise the field wiring for open or short circuit conditions. How the alarm responds to each depends on the panel as well as system zone programming, but generally speaking, an alarm views an open circuit as a fault or alarm condition, and a short circuit as a trouble or alarm condition (if armed). The purpose of EOLR's is to allow the panel to differentiate between the two conditions by looking for a known resistance.

EOLR's should be installed at the last device on the loop, electrically speaking, and not inside the control, unless special conditions are met. The benefits of EOLR's on protective zones with all concealed wiring is commonly argued by professional installers, as well as EOLR's installed inside the control unit, negating their effectiveness, as well as disabling the EOLR feature and using NC (normally closed) loops for zone definitions. The use of EOLR's is recommended and is particularly important when the field wiring is subject to damage or compromise.

A fire EOLR should always be installed at the last device and never inside the control or across the zone defined as fire, as this is an inherent safety issue.

Some equipment supports Double End of Line Resistors (DEOLR) to further differentiate between conditions that may exist on the loop.