Double End-of-Line and Tamper on One Zone

Why this matters

Double end-of-line wiring lets one pair of conductors report four things instead of three: normal, alarm, tamper and short. That is useful on devices that have a cover worth supervising, like a motion detector a burglar might spray or open during the day. It is also the wiring scheme most likely to be quietly wrong after a takeover, because the normal reading depends on one resistor and the alarm reading depends on the other. A zone with the wrong alarm resistor reads perfectly normal at the keypad and may not alarm when the device trips, and nothing tells you until somebody actually opens the cover or walks the room.

What changes from single end-of-line

Single end-of-line supervision gives the panel three bands to sort a loop into, with the resistor at the last device (see related: End-of-Line Resistor Supervision: Open, Short and Normal). Double end-of-line (DEOL) adds a second resistor so the alarm contact no longer breaks the loop open when it trips. Instead, it adds resistance. That frees "open" to mean something else: tamper, or a cut cable.

PANEL            INSIDE THE DEVICE (one device per zone)
Z5  o--------+---[RA alarm resistor]---+
             |                         |
             +------[ALARM NC]---------+
                                       |
                               [TAMPER NC switch]
                                       |
                                [RE end-of-line R]
                                       |
COM o----------------------------------+

STATE            LOOP READS
normal           RE        (alarm contact shorts RA)
alarm            RA + RE   (contact open, RA in path)
tamper or cut    open      (tamper open breaks loop)
short            near 0    (pair shorted before RA)

Walk the four states. With the alarm contact closed, current bypasses RA through the contact, so the loop reads only RE. Trip the device and the contact opens, which forces current through RA, so the loop reads RA plus RE. Lift the cover and the tamper switch opens the whole series path: open. Short the pair anywhere between the panel and the device and the panel reads a few ohms. Four bands, one pair.

Naming varies across documentation. Some makers call RE the "tamper" or "supervision" resistor because its presence is what proves the loop intact; others call RA the "alarm" resistor and RE the "end-of-line". Go by position in the circuit, not by the label in the book you happened to learn on.

The resistor values have to be exactly what the documentation says

Most panels supporting DEOL expect specific values in both positions, and many expect the same value in both, so normal reads 1 x R and alarm reads 2 x R. The bands the panel uses to sort readings are built around those values, with a gap between them. The values are per the panel's documentation; there is no number that carries from one maker to another.

Here is why near enough is not enough. The normal reading depends ONLY on RE. The alarm reading depends on RA plus RE. So:

• Wrong RE: the zone reads wrong the moment it is landed. Visible immediately, fixed immediately.
• Wrong RA: the zone reads a perfect normal, because RA is bypassed by the closed contact. The fault only exists when the device trips, which is exactly when you need it not to.

Say the panel calls for equal values, and a takeover left an alarm resistor at half the new panel's value. Normal reads 1.0 x R: fine. Alarm reads 0.5 x R + 1.0 x R = 1.5 x R. Whether 1.5 x R lands in the alarm band, the normal band or the dead gap between them depends on band edges in the documentation, which means the zone's ability to alarm is resting on a margin nobody designed for. Some panels will call it alarm. Some will call it normal. Neither is a system you can hand a customer.

Where it is supported, and how panels switch it on

DEOL is a panel feature, not a wiring trick. Three things to confirm before choosing it:

• The panel supports it on hardwired zones. Many do; some only on the onboard zones and not on expanders.
• How it is enabled. On some panels it is a per-zone option; on others it is one global supervision setting that applies to every hardwired zone on the panel. A global setting means every zone, including ones you never intended to change, now expects two resistors. Switching a panel's global setting during a service call turns every single-EOL zone into a trouble.
• How many devices it tolerates per zone. With two devices in series, each with its own RA, one trip reads 2 x R and two trips read 3 x R. Some panels treat anything above the alarm threshold as alarm; others read high values as a fault. Many documents recommend one device per DEOL zone. Follow the book for the panel in front of you.

Do not confuse DEOL with zone doubling, which also uses resistor values on one pair but uses them to tell two DIFFERENT devices apart rather than alarm from tamper on one device. Same parts in the bag, different feature.

A motion detector, metered

A family room motion detector on zone 5 of a new hybrid panel set to DEOL on its hardwired zones. The panel documentation calls for the same resistor value in both positions, written here as R. The detector has an NC alarm relay and an NC cover tamper switch, and it is the only device on the zone. The cable is a 22/4: two conductors carry auxiliary power, two carry the zone.

Account on test first, because lifting a zone and opening a tamper both send signals; see related: How to Put an Alarm Account on Test and Take It Off. The detector sits near the ceiling, so work from a stepladder set on a level floor and rated for your weight plus tools, not from furniture.

1. Wire inside the device. RA lands across the two alarm relay terminals. One zone conductor goes to one side of the relay; the other side of the relay goes to the tamper switch; the tamper's other terminal goes through RE to the second zone conductor. Fold the resistor leads so the cover cannot pinch them.

2. Meter at the panel with the zone lifted so the panel's sensing voltage does not muddy the reading. Cover on, room still, detector warmed up per the maker: the meter reads R plus a couple of ohms of wire. That proves RE is in the path and the relay is closed.

3. Trip it. A helper walks across the room. The relay opens and the reading jumps to 2 x R. That is the only reading that tests RA, and it is the one most techs skip.

4. Lift the cover. The tamper opens and the meter reads open. Cover back on: R again.

5. Land the zone and test at the keypad. Walk: the panel shows alarm on zone 5 (armed) or a fault (disarmed). Cover off: tamper on zone 5, even disarmed, which is the whole point. Confirm the central station receives both events as separate codes on zone 5, then take the account off test.

The common wiring slip shows at step 2. If the loop reads 2 x R with the room still, RA is in series with the relay instead of across it, so the panel sees a permanent alarm. If step 3 still reads R, RA is shorted out (a stray strand or a jumper across it), so the relay opening changes nothing the panel can see and the detector cannot alarm. If step 3 reads open, RA is missing altogether, and the panel will report every walk through the room as a tamper.

DEOL against a separate tamper zone

The alternative is the older one: alarm on one zone (single EOL), tamper on its own loop. On a 22/4 run with two conductors already used for power, that takes a second cable or a 22/6.

DEOL wins when zone inputs or conductors are scarce, when each device's tamper should report under that device's own zone number, and when the panel supports it cleanly on the zones in question.

A separate tamper zone wins when several devices can share one tamper loop (every siren and junction box on one 24-hour tamper zone), when you are adding tamper to an existing panel whose global setting would force a rewire of every zone, and when the next tech is likely to be someone who has only ever wired single EOL. A tamper zone is also easier to troubleshoot: its resistance is a single-EOL reading, and nothing about it depends on a second resistor value being right.

A loss to name on the separate-zone side: a shared tamper loop tells you a tamper happened somewhere on the loop, not which device. That trade is fine on sirens and enclosures; it is weaker on interior motions, where knowing which room is useful to the central station.

How to verify you got this right

For every DEOL zone, three meter readings at the panel with the zone lifted, written on the zone sheet: idle (R plus wire), tripped (2 x R plus wire, or whatever the documentation's alarm value is), cover off (open). Then the same three events confirmed at the keypad and at the central station under the right zone. A DEOL zone with only an idle reading on the sheet has not been verified, because idle never tested the alarm resistor.

On a takeover with existing DEOL, do not trust inherited resistors. Meter the tripped reading on every zone. Where the old and new panels' documented values differ, change both resistors in every device; changing only the one that made the idle reading look right is how the wrong-RA zone gets built.

References

• Panel manufacturer's installation manual (DEOL resistor values, alarm and normal bands, per-zone or global setting, devices per zone)
• Detector manufacturer's installation instructions (relay and tamper terminals, warm-up time)
• See related: End-of-Line Resistor Supervision: Open, Short and Normal; Normally Closed vs Normally Open Devices on One Zone; Security + Alarm Systems Reference