If you’ve been an installer long, you’ve no doubt seen this noted somewhere. Supervision resistors having been around for a long time and they are such an important thing. So, why do some installers still put them in incorrectly, if at all? 

An End-Of-Line resistor (abbreviated as EOL or EOLR) supervises the wiring between the controller and the end of line device (door contact, request to exit device, input push button, etc.).

When feasible, every input device should have an EOL resistor at the device. This way the controller can automatically monitor the integrity of the system against tampering. 

Intrusion systems use a “closed circuit” detection scheme where it monitors for the resistive value at the door contact. The panel normally sees the EOL resistor at the end of the detection circuit, assuming the door is closed. A short on a wire causes a trouble condition while an open indicates the door is open.

If the EOL resistor is at the contact on a door, the controller ‘sees’ the wire open when the door is opened and ‘sees’ the resistor when the door is closed.

Hacking the System

The thief who shorts wires together ‘hides’ the EOL resistor from the panel. By the panel seeing a short rather than resistive value it lets the controller know that something is going on.

The thief who cuts the wire causes an open circuit, the same as if he had opened the door. Therefore presenting as the door is ‘open’ when not actually open, also causing the end user to investigate and realize there is a problem.

As a result, if you don’t put the resistor at the true end of line you leave the system vulnerable to tampering and not indicating a fault or alarm status appropriately. Both examples above would not have reported correctly if the supervision wasn’t installed properly at the end of line.

Other Supervision

Hold-up/panic alarms and fire alarms use an “open circuit” detection scheme. The panel normally sees the EOL resistor at the end of the detection circuit. A short on the wire causes an alarm and a break in the wire causes a trouble report.

There are also more advanced systems which allow for double (and even triple) end of line supervision but at a minimum, everyone should be properly installing a single resistor at the true end of line.


Have questions?

Does your company use the EOL/EOLR? Comment below with other questions you have about these components and their application?