---
title: Send a KNX command after a period without telegrams in Link’nX
description: Monitor one or more KNX group addresses and send a value to a target when no telegram arrives during the chosen delay, with Link’nX presence detection.
canonical: https://wiki.can-nx.com/en/guides/link-function-presence/
language: en
product: Link’nX
audience:
  - administrateur
  - installateur
context: "Link’nX web: Functions, Presence detection type"
---

# Send a KNX command after a period without telegrams in Link’nX

Context: Link’nX web: Functions, Presence detection type

The **Presence detection** function type in Link’nX monitors one or more KNX group addresses. When no telegram is received on them for the chosen duration, it sends a value to a target address.

Intended for administrators and installers who know the KNX addresses to monitor and the target command. Despite its name, the function does not by itself verify that a person is present or absent: choose the sources according to the known behavior of your equipment.

## Before you start

- You know the source addresses to monitor, and the address, DPT and value of the target command.
- The timer is not accurate to the second. For an exact or very short delay, contact support before choosing this function.

## Choose the source addresses

1. In **Functions**, click **Add**, fill in **Function name** and choose **Presence detection** in **Function type**. To edit an existing function, open its **Edit** pencil.
2. In **Presence detection settings**, under **Sources**, fill in **Source address #1**.
3. To monitor another address, click **+ Add** and fill in the new row. The minus button on the right immediately removes a row from the form; it is disabled when only one source remains.

## Set the timer and the command

In **Timeout action sequence**:

1. After **If no value is received after**, enter the timer in minutes, with the suffix **min**. Enter a positive duration: for example, **0.5** corresponds to 30 seconds and **2** to two minutes. These examples are fictitious and do not set the delay suited to your installation.
2. After **Then send the value**, fill in the timeout value. For a target of the DPT 1 family, the list offers **0 (false)** and **1 (true)**; for a numeric target, enter the value suited to the command. The [output values guide](https://wiki.can-nx.com/en/guides/link-function-values/) also covers the text case.
3. After **To KNX address**, fill in the target address, then its type in **with DPT**. The target DPT determines in particular how the value field is presented: reread the value after choosing it.

Format of the source and target addresses: three levels separated by slashes without spaces, from 0 to 31, then 0 to 7, then 0 to 255. Format of the target DPT: a main number from 1 to 999, a dot, then three or four digits. **1/2/3** and **1.001** are fictitious examples; use the parameters of the installation.

## Understand the trigger

- A KNX write or response received on any of the sources restarts the timer, including when the value received is zero.
- A read request, without a value, does not restart it.
- If no new value is received during the period, the function requests the timeout value on the target. It does not repeat this command until new activity has rearmed the timer.
- The monitoring can result in a command even if no value has been received since the function was put into service.

Do not confuse the absence of telegrams with the reception of an absence value: a value received on a source, even zero, restarts the timer.

## Save the function

1. Reread the **Runtime interpretation**. The preview shows the delay in **ms**, while the entry is in minutes: one minute corresponds to 60,000 ms. Also check the value, the destination and the **Enabled** or **Disabled** state.
2. Click **Save**, or **Update** for an existing function.

The [functions guide](https://wiki.can-nx.com/en/guides/link-function-edit/) explains the confirmations and disabling.

## Check the result

After the confirmation, find the card in the **Presence detection** group and check **Delay**, **Value to send**, **Sources** and the state of the function.

## Troubleshooting

| Message | Action |
| --- | --- |
| **Each source must be a valid KNX group address** | Check all the source rows and remove empty additions |
| **Timeout must be greater than 0** | Enter a positive duration |
| **Invalid KNX group address** or **Invalid DPT format** | Check the target and its format |

For an unknown parameter or an unexpected result after the checks in the [function troubleshooting](https://wiki.can-nx.com/en/guides/link-function-help/), contact support@can-nx.com with the context, the known settings and the tests already made.