topic Re: Modbus RTU connection to multiple slaves in LabVIEW

Modbus RTU connection to multiple slaves

Sparkman2 — Thu, 23 Feb 2023 17:36:44 GMT

I can connect to a single Modbus RTU slave with no problem. However, how can I expand this to communicate with multiple slaves using the same COM port?

Re: Modbus RTU connection to multiple slaves

StevenD — Thu, 23 Feb 2023 17:43:19 GMT

Change the addresses to be different. I cannot open your code as I don't have LV 2023. You may want to save for previous version code posted here. However, if you can talk to one slave device, the address is part of the communication for Modbus RTU. You will need a way to change it on your slave device. (I've had to connect one at a time and set the address before integrating into a bigger project).

Re: Modbus RTU connection to multiple slaves

Sparkman2 — Thu, 23 Feb 2023 17:47:32 GMT

How can I change the Unit ID on the fly so that I can address another unit and keep the information separate? What I have now does well for one unit and graphs the temperature every second.

Re: Modbus RTU connection to multiple slaves

Sparkman2 — Thu, 23 Feb 2023 17:54:01 GMT

To better see what I have so far, I am attaching a Word document with the VI.

Re: Modbus RTU connection to multiple slaves

StevenD — Thu, 23 Feb 2023 18:11:19 GMT

Save for a previous version is the preferred method. Posting a png image also works. Using a proprietary Word document to insert an image is something I am not going to look at.

Re: Modbus RTU connection to multiple slaves

Sparkman2 — Thu, 23 Feb 2023 18:26:03 GMT

Didn't know what version to save as, so here is the png file. Sorry!

Re: Modbus RTU connection to multiple slaves

StevenD — Thu, 23 Feb 2023 18:57:26 GMT

Ok. It looks like you are using NI's Modbus library, but honestly I am a little confused as to why they have a yellow border. Maybe a newer feature I am not familiar with.

At any rate, you want two Create Master functions. Both have the same com port and baud rate, but with different ID's. Use the two instance wires the same as how you have done with one. If they are the same type of device and you want the same data you can bundle the instance wires and use a for loop (see the picture).

Re: Modbus RTU connection to multiple slaves

Sparkman2 — Thu, 23 Feb 2023 19:06:01 GMT

Thank you! I will try that. The yellow borders are because I unchecked "View as Icon" as it is easier on my old eyes that way. Not sure if trying to create two instances using the same COM port will cause a conflict when trying to open the second one. With what I have now, in order to see the second unit, I have to stop it, change the ID, then start it again. I have to stop it as the COM port isn't released until the While Loop completes execution. Thanks again for the assistance!

Re: Modbus RTU connection to multiple slaves

StevenD — Thu, 23 Feb 2023 19:14:52 GMT

I think I have used two devices on the same COM port. The NI Modbus Library should be smart enough to deal with it as multidrop is part of the Modbus specification (assuming it is 485 wiring, not 232). If you do have issues with it, there is another Modbus Library other people on the forums use called Plasmionique

https://www.vipm.io/package/mb_master/

Re: Modbus RTU connection to multiple slaves

cordm — Thu, 23 Feb 2023 20:29:04 GMT

You can just change the address with Set Unit ID.vi

Re: Modbus RTU connection to multiple slaves

StevenD — Thu, 23 Feb 2023 21:01:56 GMT

That seems to be the correct way. My apologies.

Re: Modbus RTU connection to multiple slaves

Sparkman2 — Fri, 03 Mar 2023 12:59:42 GMT

Do you mean that I should switch back and forth between the addresses? Read one, change address, read the next?

Thanks!

Re: Modbus RTU connection to multiple slaves

cordm — Fri, 03 Mar 2023 15:24:43 GMT

Yes, exactly. In principle, you can also set the unit ID once and create an array of Modbus Master instances.

Re: Modbus RTU connection to multiple slaves

felipefoz — Mon, 04 Dec 2023 21:33:25 GMT

@StevenD escreveu:

I think I have used two devices on the same COM port. The NI Modbus Library should be smart enough to deal with it as multidrop is part of the Modbus specification (assuming it is 485 wiring, not 232). If you do have issues with it, there is another Modbus Library other people on the forums use called Plasmionique

https://www.vipm.io/package/mb_master/

Are you aware if this is true for slaves too?

For example, multiple LabVIEW modbus slaves in the same COM port?

Regards,