End-of-line resistors (EOL resistors) play a critical role in the reliability and integrity of fire alarm systems, security systems, and other monitoring circuits.

Although they may seem like a small component, their function is essential for detecting faults and maintaining safety.

What is an End of Line Resistor?

An end-of-line resistor (EOL resistor) is a resistor installed at the end of a circuit to monitor the integrity of wiring in security, fire alarm, or signaling systems.

It provides a specific resistance value that is recognized by the control panel to confirm that the wiring is intact and the circuit is properly terminated.

EOL resistors are passive electrical components and are typically placed at the last device in a looped (supervised) circuit.

Applications of End-of-Line Resistors

EOL resistors are commonly used in the following systems:

Fire Alarm Systems
To monitor for open or short circuits in detection zones.

Intrusion Detection / Burglar Alarms
Installed in zones (door/window contacts, motion detectors) to monitor tampering or disconnection.

Emergency Notification Systems
Used to supervise audio or visual alert wiring.

Industrial Automation
To maintain the integrity of monitoring circuits for safety-critical processes.

In all these applications, the EOL resistor helps ensure that the system is not only working but also capable of detecting any faults in wiring or tampering with devices.

Why Are End-of-Line Resistors Used?

Here are the key reasons EOL resistors are implemented in supervised systems:

Circuit Integrity Check
The control panel continuously checks the resistance at the end of the circuit. If the resistor is missing, shorted, or replaced with an incorrect value, the panel will trigger a trouble or fault condition.

Tamper Detection
Prevents bypassing security sensors or alarm contacts. If someone tries to short or cut the wires, the panel will notice the change in resistance and respond accordingly.

Safety and Compliance
EOL resistors are required in most commercial fire and security systems by NFPA and UL standards to ensure safety and system readiness.

Troubleshooting Aid
Makes it easier for technicians to quickly detect line faults during installation or maintenance.

How to Select the Right End-of-Line Resistor

Choosing the right EOL resistor depends on the system design and control panel specifications. Here’s what you need to consider:

Resistance Value (Ohms)

• The most critical factor.
• Common values: 1kΩ, 2.2kΩ, 4.7kΩ, 5.6kΩ, 10kΩ, etc.
• Always match the value recommended by the manufacturer of the control panel.

Wattage Rating

• Typically low (1/4 watt or 1/2 watt).
• For most alarm and fire circuits, 1/4 W is sufficient due to low current.

Tolerance

• ±5% is usually acceptable.
• For highly sensitive circuits, a tighter tolerance (e.g., ±1%) may be required.

Physical Size and Form

• Through-hole (leaded resistors) are common.
• Some applications may use surface-mount resistors.

Environmental Considerations

For harsh environments, consider resistors with protective coating or rated for high temperature and humidity.

Practical tip: Never guess the resistor value. Refer to the system manual or panel datasheet before installation.

How It Works in a Fire Alarm Zone

Let’s say a fire alarm zone requires a 4.7 kΩ EOL resistor. Here’s how it works:

When everything is correctly wired, the control panel sees 4.7 kΩ at the end of the circuit → Normal condition.

• If someone cuts the wire or removes the resistor → Open circuit detected → Trouble signal.
• If someone shorts the wires → Zero resistance detected → Trouble or Tamper alert.

This monitoring ensures the fire alarm system will operate properly in case of an emergency.

FAQ: What is an End-of-Line Resistor?

What happens if I don’t install an end-of-line resistor?

The system will likely show a trouble condition, and in many cases, the zone won’t function properly. EOL resistors are essential for proper supervision.

Can I use any resistor value as an EOL resistor?

No. You must use the exact value specified by the control panel manufacturer. Using an incorrect resistor will result in false alarms or circuit faults.

Where exactly should I place the EOL resistor?

It should be installed at the last device in the zone wiring, not at the control panel. This allows full supervision of the entire circuit length.

Can I install multiple EOL resistors in parallel?

Generally, no—only one EOL resistor per supervised zone is allowed unless the system design specifically permits parallel resistors. Adding more can alter resistance values and confuse the panel.

Are EOL resistors reusable?

Technically yes, but it’s best to use a new resistor each time you rewire a system to ensure integrity and avoid damage from previous use.

How do I test an end-of-line resistor?

Use a digital multimeter set to the resistance (Ω) mode. Disconnect one side of the resistor and measure its resistance across both leads. Compare to the expected value.

Key Takeaways: What is an End-of-Line Resistor?

End-of-line resistors are small but powerful components that play a major role in supervising alarm and control circuits.

Whether you’re installing a fire alarm, a security system, or any supervised loop, selecting and placing the correct EOL resistor ensures your system is safe, secure, and up to code.

If you’re setting up or maintaining a system, don’t overlook the EOL resistor, it could be the difference between detecting a fault or missing a critical failure.

I get this question a lot: What is the difference between sink and source when it comes to wiring the sensors to the controller? In this article, I am going to explain the difference between the two.

The concept of sink and source

If you have wired a sensor or transmitter to a controller such as PLC, I am sure you heard the terms sinking and sourcing.

What are source and sink?

The concept of sink and source describes a current flow relationship between input and output devices in a control system and their power supply. The two terminologies apply only to DC (Direct Current) logic circuits.

A sinking digital I/O (input/output) provides a grounded connection to the load, whereas a sourcing digital I/O provides a voltage source to the load.

Let’s assume that you want to wire a field device to a controller.

If the current is flowing from the field device to the controller, we say that the field device is sourcing to the controller and the controller is sinking about the field device. and vice versa is true.

One confusion I face when explaining this concept to the customer is that we can be talking about the same thing but with different references, so if someone tells you about sink and source, always ask them what their reference is (if it is a field device or a controller).

The most important point to remember here is that in both cases you have current flowing from one device to another; you just need to figure out in which direction.

How to wire a source sensor to a controller?

The 3-wire 4-20 mA loop uses three wires to connect the field device with the controller; here the signal has its own wire, so you have one wire for the +, one wire for the -, and one wire for the signal.

The two wires (the + and the -) are used to power the field device, while the signal wire is used to carry the field device signal to the controller.

The most important thing to note here is the current move from the field device to the controller.

How to wire a sensor sink to a controller?

This is almost the same as the three-wire source type. The 3-wire 4-20 mA loop uses three wires to connect the field device with the controller; here the signal has its own wire, so you have one wire for the +, one wire for the -, and one wire for the signal.

The two wires (the + and the -) are used to power the field device, while the signal wire is used to carry the field device signal to the controller.

The main difference between the 3-wire sink and 3-wire source is that in the 3-wire sink configuration, the current signal moves from the controller to the field device.

FaQ about Sink and Source

What is the difference between sink and source?

The difference between sink and source is that in a source connection, the current flows from the field device to the controller, and in a sink connection, the current flows from the controller to the field device.

How do I know if my controller is a sink or source?

The easiest way to know if the controller is a sink or source is to check the input card; it should specify that.

If it is not clear, you can read the controller user guide, or you can give a call to the manufacturer, and their tech support should be able to tell you if the controller is a sink or source.

How do I know if my sensor or transmitter is a sink or source?

The easiest way to know if your sensor or transmitter is a sink or source is to check the wiring diagram in the user manual or give a call to the manufacturer of the field device.

In my experience, most field devices (sensors and transmitters) come with deep switches that you can use to change them to be a source, sink, or loop.

Can I wire a sink transmitter to a sink controller?

No, you cannot wire a sink transmitter to a sink controller. The reason for this is that both units will be expecting to draw current from the circuit. This will lead to the wrong signal being sent, and eventually, the units might not power up.

Can I wire a source transmitter to a source controller?

No, you cannot wire a source transmitter to a source controller; both units will be providing current to the system, and this will lead to the wrong reading, and the unit might get damaged. Do not do this.

The best way to wire is to set one unit as a sink and one as a source.

Conclusion

We have analyzed the difference between sink and source when it comes to wiring industrial transmitters to the controllers and also answered some frequently asked questions about the subject.

If something is not clear or you have any further questions, please leave them in the comment section below.