Where should the EOL resistor be placed when installing NC door contacts?

When installing normally closed (NC) door contacts, the end-of-line (EOL) resistor should be placed in series at the device. This configuration is essential for a few reasons.

Firstly, placing the EOL resistor in series with the circuit allows the resistor to monitor the integrity of the wiring. If the circuit is tampered with—either through cutting the wires or by disconnecting the contacts—the resistor will create an open circuit, allowing the control panel to register a fault.

Additionally, when the door contact is closed (which is the normal operating condition), the EOL resistor completes the circuit and ensures the proper voltage is delivered to the control panel. This setup is important for signaling a secure state and for the control panel to function accurately, providing reliable monitoring of the door’s status.

In contrast, placing the resistor in parallel would not properly register changes to the circuit's condition. This would lead to potential undetected tampering and insufficient signaling to the control panel, which defeats the purpose of the EOL resistor in alarm systems.