---
title: Test a Modbus register read and troubleshoot monitoring in Boss’nX
description: "Live test error, request timed out, negative or offset Modbus value, readings missing from the history: causes and the checks to run in Boss’nX."
canonical: https://wiki.can-nx.com/en/guides/boss-modbus-config-help/
language: en
product: Boss’nX
audience:
  - administrateur
  - installateur
context: "Boss’nX, supervision on a computer: Data logging, Modbus registers tab, Live test"
---

# Test a Modbus register read and troubleshoot monitoring in Boss’nX

Context: Boss’nX, supervision on a computer: Data logging, Modbus registers tab, Live test

**Live test** greyed out or failing, timeout, inconsistent value, readings missing from the history, empty list or refused save: here is how to test a Boss’nX Modbus monitoring configuration and solve the most common problems.

For administrators and installers with **Manage components**, in the relevant supervision, on a computer. Open **Menu → Data logging → Modbus registers**, then click the configuration to check. For the fields and saving, see [the configuration guide](https://wiki.can-nx.com/en/guides/boss-modbus-config/).

## How do I test a reading with Live test?

1. In the form, check **Gateway**, **Register address**, **Word length**, **Function code**, **Data type**, **Unit ID**, **Scale factor**, **Swap Bytes** and **Swap Words**.
2. To read a register, use the read code **3** or **4** confirmed for the device, never a write function.
3. Click **Live test**. The button shows **Connecting…** while waiting.
4. If a result arrives, read **Value** and **Timestamp**. The time shown is when the interface received the result, not the device’s clock.
5. Check that the value is plausible with the intended unit and scale.

The test uses the form’s current parameters, even before they are saved, and saves nothing. After any change to the parameters, run a new test: the previous result applied to the old settings. To keep a change, then click **Save** and reopen the row. A successful test checks the reading at that moment; the continuity of the history is checked over the relevant period.

## Why is Live test greyed out?

The button stays greyed out as long as no valid gateway is selected or a test is already in progress. Select the intended gateway or wait for the test to finish. If the gateway list is empty or does not match the installation, have [the declared Modbus access](https://wiki.can-nx.com/en/guides/boss-modbus-gateway-create/) checked.

## What should I do if the test fails or times out?

**Error during test.** means the test connection could not be established. **Request timed out.** appears when no answer arrives within ten seconds. These messages alone point neither to a wrong register nor to a faulty device: check in this order.

1. Check the open supervision and your connection to Boss’nX.
2. Check the selected gateway and have someone confirm that it is enabled and matches the right network access, using [Modbus gateway management](https://wiki.can-nx.com/en/guides/boss-modbus-gateway-manage/). Do not change its IP address at random.
3. Compare the read parameters with the device’s confirmed table: address and addressing convention, read code, Unit ID, length and type. For a thirty-two-bit register, check **Word length** in particular.
4. If an input error is established, correct only the parameter concerned and run the test again.

If these checks are done and the test still fails, contact support.

## Why is the value too large, negative or offset?

These differences often come from a decoding parameter that does not match the device. First check the signed or unsigned type, the **Scale factor** and **Word length**, then have the intended byte or word swaps confirmed. The value alone does not tell you which of these causes is involved: do not enable the swaps one after the other without a reference.

## Why are the readings missing from the history when the test responds?

Reopen the configuration to check the saved parameters, then check the right period and the right source in the relevant viewing screen. The planned monitoring rate is sixty seconds, subject to communication and acquisition. If the problem persists, contact support with the test result and the period concerned. Do not delete the configuration to try to recreate the history.

## Why does an imported component datapoint not follow the monitoring change?

[The import of Modbus metering configurations](https://wiki.can-nx.com/en/guides/boss-import-modbus/) creates a one-time copy. Check the datapoint’s settings with [the component Modbus datapoint guide](https://wiki.can-nx.com/en/guides/boss-point-modbus/) and save the component: saving the monitoring configuration does not update it.

## What do the list, save or deletion messages mean?

| Message or symptom | Useful checks |
| --- | --- |
| **No Modbus configurations.** | Clear the search and remove the gateway filter by clicking its chip again. The search covers the name, the register and the type. |
| **No gateway** above a group | These configurations are attached to a gateway that is missing from the list of Modbus gateways. Check the Modbus access they should be attached to, and check what already exists before creating a new gateway. |
| **Unable to load Modbus data.** | The empty list that follows comes from the loading error, not from missing configurations or a deletion. Check the connection and the supervision, then reopen the **Modbus registers** tab. |
| **This field is required.** | Enter a non-empty **Name** and check the **Gateway**. Also check the numeric fields flagged by the browser. |
| **Error while saving.** | Keep the useful parameters and check the list before trying again: the save has not necessarily succeeded. Avoid creating a duplicate. |
| **Error while deleting.** | Reopen the list and check whether the configuration is still present before starting another deletion. |
| **Data logging** missing from the **Menu** | Have **Manage components**, the open supervision and the **Integration** family of the **Menu**, which may be folded, checked. Mere viewing access does not give this permission. |

## Can I read a binary state or send a command from monitoring?

Modbus monitoring is used to read registers. The **Function code** list also contains **1 — Read Coils**, **2 — Read Discrete Inputs**, **5 — Write Single Coil** and **6 — Write Single Holding Register**, but the presence of a write code does not make monitoring a command tool.

- To read a binary state, have support confirm the encoding and the configuration suited to the device.
- To send a command, configure [a Modbus command datapoint](https://wiki.can-nx.com/en/guides/boss-point-modbus/) in a component.

Addresses, conversions and model-specific parameters must be confirmed with the installer or support. If the history must be kept after a register change, a duplication or a deletion, have the expected effect clarified before acting.

## Contact support

If the difficulty persists, write to support@can-nx.com with: the product, the supervision, the name of the configuration and of the gateway, the requested action, the last message shown, the test result and the period concerned, and the checks already made. A screenshot of the relevant fields without sensitive data can help. Do not send any password, access key or confidential log.

French labels: **Test en direct**, **Connexion…**, **Valeur**, **Horodatage**, **Erreur lors du test.**, **Délai d'attente dépassé.**, **Impossible de charger les données Modbus.**, **Erreur lors de l'enregistrement.**, **Erreur lors de la suppression.**, **Ce champ est requis.**, **Sans passerelle**.