Reading EOL resistors with a meter

How the panel reads the resistor

Most access control and intrusion panels, including Bosch panels and Mercury-based controllers, supervise an input by sending a reference voltage out on it through a resistor inside the panel. The EOL resistor at the device and the resistor inside the panel form a voltage divider, and the panel measures the voltage that results at the input. When the expected resistor is in place, the input sits at a known voltage. When the reading moves above or below that window, the panel reports alarm, trouble, or tamper depending on which way it moved and how far.

As an illustration only: a panel that applies 5 volts through an internal 1 kΩ resistor to an input with a 1 kΩ EOL resistor would see about 2.5 volts at the input. If the contact opens and adds a second resistor, the voltage rises. If the pair is shorted, it falls toward zero. The actual reference voltage, internal resistance, and thresholds are specific to each manufacturer and model.

Why the panel interferes with a meter

A meter on the resistance setting works by sending its own small current through the circuit and measuring the voltage across its probes. If the pair is still connected to the panel, the panel's reference voltage is on the same conductors, and the resistor inside the panel is part of the circuit the meter is reading. The result is a number that is not the EOL resistor: a value that drifts, a reading that is too high or too low, or on some meters a negative value or an error.

The same thing happens on a smaller scale with anything else connected across the pair: another device on the same input, a second contact, or a person's hands on the probes.

Measuring without interference

1. Lift the pair off the panel input. Remove both conductors from the terminals, or at minimum one, so the panel's voltage and internal resistor are out of the circuit. Never measure resistance on a powered input.
2. Measure across the two lifted conductors. This reads the field circuit as the panel would see it, including the cable.
3. Know which resistors are in the circuit in the state being measured. A closed contact bypasses any resistor across it, so the reading depends on whether the door is open or closed. The EOL supervision wiring reference shows what each arrangement reads in each state.
4. To read one resistor by itself, isolate it at the device by lifting one of its leads. Anything still connected around it adds a path and changes the reading.
5. Keep fingers off the metal probe tips and the resistor leads. A body in parallel with a high-value resistor, such as 22 kΩ, pulls the reading down noticeably.
6. On low readings, touch the probes together first and note what the meter reads with the leads alone. Subtract it from readings under about 100 ohms.
7. Allow for the cable. 22 AWG copper adds roughly 32 ohms per 1,000 feet of run, counting both conductors. On a 1 kΩ or larger resistor that is usually small enough to ignore.
8. Reconnect the pair to the same terminals and confirm the panel reports the input normal.

Checking the input with the panel connected

With the pair connected and the panel powered, the input can be checked on the meter's DC volts setting instead of resistance. Measure across the input terminals and compare the voltage to the manufacturer's table for normal, alarm, and trouble. This reads the circuit the way the panel reads it, and it is useful for confirming a zone without disconnecting anything. The expected voltages come from the panel documentation.

Reading the display

A meter shows the same value in different ways depending on its range. A reading of 4.70 kΩ and a reading of 4700 Ω are the same, and both are a 4.7 kΩ resistor. Auto-ranging meters move the decimal and the unit as the value changes, so read the unit symbol, not just the digits: Ω is ohms, kΩ is thousands of ohms, and MΩ is millions. OL means open, or over the range the meter is set to.

Resistors are identified by the nearest standard value, not by an exact match. A 4.7 kΩ resistor at 5 percent tolerance can read anywhere from about 4.47 kΩ to 4.94 kΩ, and cable adds a little more. A reading of 5.0 kΩ is most likely a 5.1 kΩ part, because 5.1 kΩ is the standard value closest to it.

Meter reading to resistor value

Common EOL values, with the range a 5 percent part normally reads and the color bands on a four-band part. Ranges for neighboring values overlap at 5 percent. When a reading falls in the overlap, take the closer nominal value and confirm against the color bands or the panel documentation.

Readings from dual EOL circuits

On a dual EOL circuit in alarm, the meter reads both resistors added together, which can look like a single resistor of a different value. A 1 kΩ and 1 kΩ pair reads 2 kΩ in alarm, the same as a single 2 kΩ resistor. Know the circuit before naming the part.