adding device telemetry with undetermined number of values

Avigdor 40

Hello,

I want to create a component to hold MODBUS register values and also be able to configure registers to be read.

How can I configure a component like that without knowing the amount of registers in advance?

Manas Mohanty 1,610 Reputation points Microsoft External Staff

2025-03-07T08:05:09.16+00:00

Hi Avigdor

Hope the below pointers from Sander van de Velde | MVP addressed your requirement.

Just wanted to emphasize that while you are accumulating information on all registers data, you can go ahead creating device template with critical parameters and update it later once you have accumulated all data.

Reference on modifying Device template

Thank you.
Manas Mohanty 1,610 Reputation points Microsoft External Staff

2025-03-10T06:18:14.5166667+00:00

Hi Avigdor

We have not heard from you. Hope the below pointers were useful to you.

Thank you.

1 answer

Sander van de Velde | MVP 35,336 Reputation points MVP

2025-03-06T12:00:32.7833333+00:00

Hello @Avigdor ,

welcome to this moderated Azure community forum.

working with the modbus protocol in industrial (Azure) IoT is a common scenario.

You have added the tag Azure IoT Central which is the SaaS solution based on Azure IoT PaaS building blocks like the Azure IoT Hub.

Azure IoT Central connects to devices only using the way that device can connect to the Azure IoT Hub. This is usually done via one of the SDKs of via the MQTT Protocol.

Using Modbus (TCP) to connect to the cloud is a really not recommended. Modbus has no security.

One way to connect a device speaking Modbus is buying a device (I have worked with Advantech Wise IO modules but there are many other devices available) or building a device that can connect to the Azure IoT Hub and communicate Modbus.

Another solution is to make use of Azure IoT Edge, an edge solution you can put on top of an IPC, living in the local network and having a secure outbound connection to the IoT Hub.

This can work with both Modbus TCP as Modbus RTU.

This supports two way communication. See the examples on the related blog posts.

If the response helped, do "Accept Answer". If it doesn't work, please let us know the progress. All community members with similar issues will benefit by doing so. Your contribution is highly appreciated.
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Avigdor 40 Reputation points

2025-03-06T12:29:47.0033333+00:00

Hello Sander,

Thank you for the answer, I will clarify the case:
I have an IoT device that communicates with MODBUS RTU.
The device functions as master and communicates successfully with MODBUS slaves.

What I want to achieve is to send to the IoT device which registers to read from the MODBUS slave, and to be able to show the results in the azure IoT central.

The amount of registers and configuration are not known in advance, so i can't just add capability for each register in the modbus.

Avigdor 40 Reputation points

2025-03-06T13:35:29.8566667+00:00

Hello Sander,

Thank you for the response. let me clarify my case:
I have an IoT device that in one hand communicates in MODBUS RTU as master, and in the other hand communicates with Azure IoT central.

I want to be able to send properties from Azure IoT central to configure the IoT device to know which registers to read. and to set properties that will hold the results.

I don't know in advance how many or type of registers to read, and this makes it difficult for me to understand how to implement it on Azure IoT central.

Sander van de Velde | MVP 35,336 Reputation points MVP

2025-03-06T19:11:09.9066667+00:00

Hello @Avigdor ,

I expect you know how to program an Azure IoT device via the Azure IoT Device SDK and connect it to an Azure IoT Central application.

You should be able to send commands via a method and/or properties. An Azure IoT Hub supports this as Direct methods/Desired properties on the device twin.

The Modbus protocol is based on a pull mechanism way of communicating.

You need to know the registers and coils in advance. These can be found in the specifications of the device you have.

The Modbus protocol has no discovery mode on which registers and coils you can expect values to be available.

You can be creative with looping through the coils and registers but this puts an extra load on the protocol usage.

If you can switch to another protocol, check out OPC-UA a much richer protocol with node browsing support.

Avigdor 40 Reputation points

2025-03-09T07:45:11.9766667+00:00

Thank you Sander,

I was hoping to find a user friendly way to configure the device to know those registers.

i.e. creating a form that will update the device which registers it needs to read. I don't need discovery mode. what I need is the device to be notified which registers to read once, then the device holds them in its memory reads them periodically and once in a while sends the results to azure.

e.g. tell the device to read register 1 to 10 in slave id 1.

What I'm looking for is the flexibility for updating the device with registers.

Sander van de Velde | MVP 35,336 Reputation points MVP

2025-03-09T19:07:19.0166667+00:00

Hello @Avigdor ,

if your device is able to react on Azure IoT Central commands (which are basically Azure IoT Hub device direct methods), you are flexible to send a flexible request.

You could send a string like 'slave1;40001-40010' which is a normal Modbus register range for a certain slave.

But you need to be able to parse this string using custom code on your device.

Direct methods can even be JSON requests but I have no experience with the IoT Central interface so experiment with it.

Regarding the client code, check out this example in Python or this one in C#.

You can also try writable properties, these are Azure IoT Hub device twin desired properties.

Again, play with the UI and check your code in on how to read and react on Desired property changes.

If the response helped, do "Accept Answer". If it doesn't work, please let us know the progress. All community members with similar issues will benefit by doing so. Your contribution is highly appreciated.

Avigdor 40 Reputation points

2025-03-09T22:14:07.97+00:00

Thank you Sander,

with regards to updating the device, I understand. but lets say I have set the device to read slave 1 registers 40001-40010, as far as I see, to show their value I configure the registers as capabilities. is there a way to do that in a scalable fashion.

I want to know if there is an option, when a device sends capability with a name that is not configured in advance that azure IoT central will auto create that capability in the device template.
e.g. the device is configured to read registers 40001-40010, it sends them with the name "holding_reg_1" to "holding_reg_10" and azure IoT central auto creates capabilities "auto_holding_reg_1" to "auto_holding_reg_10".

Is this possible?

Sander van de Velde | MVP 35,336 Reputation points MVP

2025-03-10T07:05:18.7766667+00:00

Hello @Avigdor ,

The User Interface of Azure IoT Central is not that advanced.

The telemetry /device message template can work with arrays but you need to be creative yourself.

If you need more flexibility, check out the Azure IoT Hub where you have full control. If you combine it with Azure Data Explorer, you have full control over the representation of data.

Check out this (almost) free example.
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