Modbus I/O Server: Can't Communicate Between Master and Slave VI

Updated Dec 29, 2022

Issue Details

I have tried running my master and slave VIs on the same computer and on different computers, but I cannot communicate between them. There are no errors, but there is no communication between them.


The most common issue we run into when this occurs is when the modbus ports are already being utilized by another program, or another deployed shared library. Most standard modbus protocols utilize Port 502.

Each time you deploy a Modbus I/O Server it remains running in the background regardless of if you close your vi or project. In order to open up the TCP port for your current Modbus I/O Server you are trying to deploy you'll need to close any previously running servers.

1. You can see which Modbus I/O Servers and/or Shared Libraries that are deployed by using the NI Distributed Manager.

2. Select the previously deployed Modbus I/O Server and click Stop Process.


Additional Information

There are still several other factors that could be prohibiting communication from a Master and Slave. This is just one scenario that may be causing lack of communication.

Any deployed Shared library and/or Modbus I/O Server that you deploy remains running in the background regardless if the project/vi is up and running or not. If you have multiple Shared Libraries or Modbus I/O servers running, only the first deployment will establish proper communication if they are both targeting the same port.