---
title: "Boss’nX Modbus datapoint: test the reading and fix errors"
description: "Test the reading failing, timeout, save refused, wrong value or command with no effect: causes and fixes for a Modbus datapoint in a Boss’nX component."
canonical: https://wiki.can-nx.com/en/guides/boss-point-modbus-help/
language: en
product: Boss’nX
audience:
  - administrateur
  - installateur
context: "Boss’nX on a computer: Modbus datapoint in the component editor, Test the reading button"
---

# Boss’nX Modbus datapoint: test the reading and fix errors

Context: Boss’nX on a computer: Modbus datapoint in the component editor, Test the reading button

**Test the reading** failing, timeout, **Create** or **Save** blocked, missing or inconsistent value, command with no effect, no-signal alarm: here is how to check a Modbus datapoint of a Boss’nX component and fix the most common errors.

For administrators and installers with **Manage components**, in the component editor, on a computer. The datapoint’s fields are described in [configuring a Modbus datapoint](https://wiki.can-nx.com/en/guides/boss-point-modbus/).

## How do I test the reading of a Modbus datapoint?

1. Check **Feedback**, **Modbus gateway**, **Unit ID**, **Feedback register**, **Function code (read)** and **Data type**. Also review **Scale factor** and the **Swap bytes / Swap words** options according to the device.
2. Click **Test the reading**. While waiting, the button shows **Testing…**.
3. If a value appears under the button, compare it with the measurement expected for this datapoint.
4. To keep the configuration, then click **Create** or **Save**, and reopen the datapoint to review its settings.

The test uses the values currently being entered and checks a reading. It replaces neither the save validations nor a trial of the command.

## What do the Test the reading messages mean?

| Test message | Useful next step |
| --- | --- |
| **Select a gateway first.** | Select the intended gateway in **Modbus gateway**. If it is missing, check the supervision and [the gateway declaration](https://wiki.can-nx.com/en/guides/boss-modbus-gateway-create/). |
| **Set a feedback register first.** | Fill in the confirmed read address: the test reads this register, not the command register. |
| **No response from the gateway (timeout).** | No response arrived within the test’s time limit, about ten seconds. Check the context and the confirmed settings; this message alone does not point to a device failure. |
| **Test failed: gateway unreachable.** | Check your connection and the selected gateway, then have its reachability checked. This message does not let you attribute the cause to a specific register, cable or device. |

After a relevant correction, run the test again. If the same failure persists with the confirmed settings, contact support rather than making more register or conversion changes.

## Why does Create or Save stay blocked?

Read the errors shown under the datapoint concerned, then correct the corresponding fields.

| Message | Correction |
| --- | --- |
| **Modbus gateway required** | Choose the correct Modbus gateway for the datapoint. |
| **Unit ID must be between 1 and 247** | Fill in the confirmed identifier, an integer between **1 and 247**. |
| **Feedback register required** | Complete the register intended for **Feedback** or **Alarm**. |
| **Command register required** | Complete the write register, or uncheck **Cmd** if the datapoint should only read. |
| **Poll interval must be at least 5 s** | Enter at least **5 seconds**, according to the rate suited to the installation. |
| **Delay too short: minimum 3× the poll interval…** | Set **No signal — delay (s)** to at least three times the polling interval. Fictitious example: polling **10 s**, delay **30 s minimum**. Keep the polling interval suited to the installation rather than shortening it to get around this rule. |

A **Value list** must also have at least one option, with a numeric value and a non-empty **Label**. For general errors or a form that stays open without a clear result, follow [saving the component](https://wiki.can-nx.com/en/guides/boss-component-edit/). Check the list and the values actually saved before starting a new creation.

## Why does the saved datapoint show no value?

Reopen the datapoint and check **Feedback**, the gateway, **Unit ID**, the register and its options. Run **Test the reading**: if it fails, follow the message obtained in the table above. If the test returns a value but the card still does not show the expected result, report this difference to support.

## Why is the value read inconsistent?

Compare the register, the function code, the type, the factor and the swaps with the table confirmed by the installer. Also check the expected unit. Correct only the setting whose error is established, without changing the others at random to make a number match.

## Why does a Modbus command have no effect?

Check the saved settings, in particular **Cmd**, the command register and its function code. After a type change, the write code may have been suggested again. A successful reading or a send confirmation does not prove that the device accepts the write, nor that the hardware has changed state: watch the equipment or its feedback. Have the device’s write capabilities confirmed and contact support if the problem persists.

## What should I do about a no-signal alarm?

Check the saved polling interval and delay, then the **Test the reading** result. A disabled Modbus gateway stops polling: check its **Enabled** box in [Modbus gateway management](https://wiki.can-nx.com/en/guides/boss-modbus-gateway-manage/). Increasing the delay does not restore missing communication. To understand a persistent alarm, contact support with the durations and the known observations.

If the expected alarm does not appear, check **Pending installation**: as long as this marking is active, no alarm is created for the datapoint.

## The test responded: is the datapoint saved?

No. **Test the reading** reads the register with the form’s settings, without saving anything or sending a command. Use **Create / Save**, then reopen the component to check.

## Why don’t Min, Max and Pas appear?

Check that **Cmd** is checked, **Alarm** unchecked and **Format d'affichage → Numeric** selected. A read-only datapoint, a value list or a boolean datapoint offers other settings. A datapoint read with code **1** or **2**, or commanded with code **5** without a read register, is treated as boolean.

## Should I add or subtract 1 from the address in the device manual?

Have the correspondence between the manufacturer’s notation and the address to enter confirmed: no universal offset applies. If in doubt, contact support.

## Contact support

If the problem persists, write to support@can-nx.com with the supervision, the component, the datapoint’s **Label**, the useful settings, the current symptom and the tests already done. Distinguish an earlier message from the latest result observed. Add a screenshot without sensitive data if useful; no password or access key is needed.

French labels: **Tester la lecture / Test en cours…**, **Choisissez d'abord une passerelle.**, **Renseignez d'abord un registre retour d'état.**, **Pas de réponse de la passerelle (délai dépassé).**, **Échec du test : passerelle injoignable.** The header functions are **Cmd / Re.Etat / Alarme**; use **Re.Etat** for reading. The alarm delay is **Absence de signal — délai (s)** and the polling interval **Intervalle de scrutation (s)**.