---
title: Compare two KNX measurements in a Boss’nX component alarm
description: Trigger a Boss’nX alarm when one KNX measurement is above, below or different from another, such as flow and return temperature, with operator and delay.
canonical: https://wiki.can-nx.com/en/guides/boss-point-alarm-comparison/
language: en
product: Boss’nX
audience:
  - administrateur
  - installateur
context: "Boss’nX on a computer: component editor, KNX alarm datapoint in Comparison mode"
---

# Compare two KNX measurements in a Boss’nX component alarm

Context: Boss’nX on a computer: component editor, KNX alarm datapoint in Comparison mode

A Boss’nX comparison alarm triggers when the measurement of a KNX datapoint A is greater than, less than, equal to or different from that of a datapoint B of the same component for the chosen delay, for example a flow temperature higher than the return temperature. It is set up on a dedicated alarm datapoint, in the component editor.

For administrators and installers, on a computer. To apply a comparison to several datapoints selected by tags, follow [tag-based alarm rules](https://wiki.can-nx.com/en/guides/boss-tag-alarms-comparison/) instead: it is a different path.

## Before you start

- Your account has **Manage components**.
- The two KNX datapoints to compare are in the same component. Each one has a **Feedback** address filled in and a DPT suited to its measurement: check [their KNX configuration](https://wiki.can-nx.com/en/guides/boss-point-knx/).
- Their units and values are comparable. Check this on the actual configuration: two similar labels are not enough to establish it.
- This guide describes a simple comparison, with no offset between the measurements: **Hysteresis 0**. To add a margin or an offset, or to compare different protocols, have the suitable configuration confirmed by support; do not infer the setting from the displayed help alone.

## Create the comparison datapoint

1. Open **Components** in your bar or through **Menu → Components**, then the **Edit component** pencil. See [the common form](https://wiki.can-nx.com/en/guides/boss-component-edit/).
2. Click **Add point**, choose **KNX**, fill in its **Label** and a suitable numeric, non-Boolean **DPT**.
3. Check **Alarm**, then choose **Level / Severity**.
4. In **Trigger**, select **Comparison**. The measurements compared are those of A and B: the address and DPT fields of this new datapoint disappear.
5. In the header of this new datapoint, uncheck **Feedback** and keep **Alarm** checked; **Cmd** stays unchecked.
6. Under **Datapoint A (source)**, open **— Select —**. Search by label or address with **Search datapoint...**, then click the result after checking its full address.
7. Choose the operator in the central list: greater than ( > ), greater than or equal to ( >= ), less than ( < ), less than or equal to ( <= ), equal to (**==**) or not equal to (**!=**).
8. Under **Datapoint B (comparison)**, select the other datapoint in the same way. The two references must be different: the datapoint chosen on one side is no longer offered on the other.
9. Enter **Hysteresis 0** for the simple comparison, then **Trigger delay (min)** in minutes, with a positive or zero value.
10. Leave the last field and review the two names, the addresses, the operator and the delay.

Fictitious example: A **Flow temperature**, operator greater than ( > ), B **Return temperature**, **Hysteresis 0**, delay **2** minutes. The condition is “A greater than B”; when they are equal, it is not met. The values received from both datapoints must allow the condition to be evaluated throughout the delay.

## Understand the return to normal

The form shows **Auto-reset when condition is no longer true.** This mode does not offer a separate reset delay. Do not present the return or the notification as instant; for a precise requirement on the return timing, contact support.

The comparison datapoint refers to A and B: it does not need a third feedback address and creates no command on these two datapoints.

## Check the result

1. Click **Create** or **Save**. Before saving, **Back** discards the component’s pending changes.
2. Reopen the component and check the two references and the condition.
3. Check that both measurements are currently received, then test the alarm on the installation: saving alone does not test the comparison.

## Troubleshooting

| Message or situation | What to do |
| --- | --- |
| **No result** | Try the label or the address. Check that the datapoint is in this component with its feedback address filled in: the list only offers these datapoints, not all the datapoints and protocols of the supervision. For a missing BACnet or Modbus datapoint, do not enter a fake KNX address; contact support with the two measurements you want. |
| **Feedback address required** on the comparison datapoint | Check that **Feedback** and **Cmd** are unchecked on this new datapoint, with **Alarm** checked and **Comparison** selected. Do not change the functions of the two source datapoints. |
| Reference missing or identical after a change, for example **Datapoint A not found** or **Both datapoints must be different** | Choose two different datapoints again. Before deleting a datapoint used as a reference, check the alarms that depend on it. |
| Comparison expected but no alarm | Check the current values of the two sources, the operator and the delay, then [the filters, standby and other checks](https://wiki.can-nx.com/en/guides/boss-point-alarm-help/). Do not make repeated changes to thresholds or DPTs. |

If the blocking issue persists after these checks, contact support@can-nx.com with the component, the two datapoints, the operator, the delay and the results obtained.

French labels: **Déclenchement → Comparaison**, **Datapoint A (source)**, **Datapoint B (comparaison)**, **— Sélectionner —**, **Rechercher un datapoint...**, **Aucun résultat**, **Effacer**, **Hystérésis**, **Délai de déclenchement (min)**, **Retour automatique quand la condition n'est plus vraie.**, **Adresse de retour requise**, **Datapoint A introuvable**, **Les deux datapoints doivent être différents**, **Enregistrer**.