EOL supervision wiring
How an end-of-line resistor is wired on a supervised access control or security input, what each arrangement can detect, and what a meter reads in every state.
Why an input is supervised
An unsupervised contact gives the panel two readings, closed and open. It cannot tell a closed door from a jumper across the wires, or an open door from a cut wire. A resistor at the device gives the panel a known value to look for, so a reading that is too low or too high means something other than the door changed. Where the resistor sits, in line on one leg or across the two conductors, decides which faults the panel can see.
The resistor belongs at the device, at the far end of the pair. A resistor at the panel terminals supervises nothing beyond those terminals.
Series resistor, in line on one leg
Wired in line on one conductor, in series with a contact that is closed when the input is secure. Common on door position switches.
| State | What happens | Meter reads |
|---|---|---|
| Secure | Contact closed, current flows through Rs | Rs |
| Alarm | Contact opens | Open (OL) |
| Wires shorted | Short bypasses Rs | Near zero |
| Wire cut | Circuit opens | Open (OL) |
What it achieves: the panel can see a short, so a jumper across the pair does not defeat it. What it cannot do: a cut wire reads the same as an alarm.
Parallel resistor, across the conductors
Wired across the two conductors at the device, with a contact that is open when the input is secure and closes in alarm.
| State | What happens | Meter reads |
|---|---|---|
| Secure | Contact open, current flows through Rp | Rp |
| Alarm | Contact closes and shorts the pair | Near zero |
| Wires shorted | Same as alarm | Near zero |
| Wire cut | Circuit opens | Open (OL) |
What it achieves: the panel can see a cut wire. What it cannot do: a short reads the same as an alarm.
Dual EOL, series and parallel together
One resistor in line on the loop (Rs) and one across the contact (Rp), with a contact that is closed when secure. When the contact is closed it bypasses Rp, so the meter sees Rs. When it opens, the meter sees Rs and Rp added together.
| State | What happens | Meter reads |
|---|---|---|
| Secure | Contact closed, bypassing Rp | Rs |
| Alarm | Contact opens, Rp joins the circuit | Rs + Rp |
| Wires shorted | Short bypasses both | Near zero |
| Wire cut | Circuit opens | Open (OL) |
What it achieves: four distinguishable states on one pair. Secure, alarm, short, and cut all read differently, so the panel can separate a real alarm from tampering with the wiring.
Worked example: the GRI 11319 dual EOL pack contains a 22 kΩ and a 4.7 kΩ resistor. The panel's documentation decides which one goes in line and which goes across the contact. Wired with 4.7 kΩ in line and 22 kΩ across the contact, the meter reads 4.7 kΩ secure and 26.7 kΩ in alarm. Wired the other way, it reads 22 kΩ secure and 26.7 kΩ in alarm. The pack does not say which is correct for a given panel. The panel documentation does, and a meter confirms what was actually installed.
Normally open devices on a dual EOL input are wired differently from the arrangement above, and the arrangement varies by panel. Use the panel's wiring diagram.
No resistor
| State | Meter reads, contact closed when secure | Meter reads, contact open when secure |
|---|---|---|
| Secure | Near zero | Open (OL) |
| Alarm | Open (OL) | Near zero |
There is no supervision. A jumper or a cut wire can make the input read secure.
Several sensors on one pair
Some door circuits put the door position switch, request-to-exit, and tamper on a single pair, with each sensor switching a different resistor value into the circuit. Every combination of active sensors produces a distinct reading, and the panel decodes which sensors are active from the total. The values and arrangement are specific to the panel or the resistor pack, so these circuits are identified against the manufacturer's chart rather than by rule. Record the reading for each sensor state so it can be compared to that chart.
Line modules are not resistor packs
Some manufacturers supervise door inputs with an electronic line module rather than resistors, such as the Hirsch MELM and DTLM series. These modules report sensor states to the controller electronically. A resistance meter does not read them the way it reads a resistor pack, and readings taken across one do not identify how it is configured. Wire and test them to the manufacturer's documentation.
Reading a circuit with a meter
- Disconnect the pair from the panel. The panel places voltage on a supervised input, and a resistance reading taken with the panel connected is not reliable.
- Set the meter to resistance.
- Measure across the two conductors with every sensor secure, then change one sensor at a time and measure again: open the door, press the request-to-exit, trigger the tamper.
- Compare how the reading changes to the tables above.
- Allow for resistor tolerance, commonly 5 percent, and the resistance of the cable. 22 AWG copper adds roughly 32 ohms per 1,000 feet of run, counting both conductors.
- Record the readings on the door's cut sheet.
The meter readings reference covers measuring without interference from the panel and turning a reading into a resistor value.
How to use
- Find the arrangement that matches the device and the panel documentation, or start from the meter readings and work back to the arrangement.
- Each table lists what a meter reads in each state with the pair disconnected from the panel.
- For a circuit already installed, the meter readings reference explains how to measure it without interference and how to name the resistor from the reading.
Assumptions
- The resistor value and the arrangement come from the panel documentation. This page explains arrangements; it does not choose one for a zone.
- Readings assume the pair is disconnected from the panel and the meter is on a resistance range.
- Near zero means a reading within cable and contact resistance, commonly under 10 ohms on a short run.
- Fire alarm circuits are outside the scope of this page.
- Manufacturer data, the AHJ, and current applicable code govern.
FAQ
- Why does the resistor go at the device instead of the panel?
- Supervision only covers the wiring between the panel and the resistor. A resistor at the panel terminals leaves the entire run unsupervised.
- Can I read the circuit with the panel still connected?
- Not on a resistance range. The panel places voltage on a supervised input, so the meter reading is not a true resistance. Disconnect the pair first.
- Why does my reading differ from the resistor value?
- Resistor tolerance and cable resistance both move the reading. A 4.7 kΩ resistor at 5 percent can read anywhere from about 4.47 kΩ to 4.94 kΩ before the cable is added.
- Does a dual EOL pack tell me which resistor goes in line?
- No. The panel documentation does. A meter confirms what was installed.
Related references
Disclaimer
Field estimate only. Manufacturer data, the authority having jurisdiction (AHJ), and current applicable code govern material and whether a circuit is energized. See the disclaimer.