MODBUS client is not connecting

MODBUS client is not able to connect to the JNIOR

The MODBUS server running on the JNIOR is a separate application from the Operating System. It does not run by default when the JNIOR is shipped. It is installed by default. It must be activated by the end user before the the MODBUS connection can be successful.

To activate it go to the DCP, click on the Configuration tab. Half way down the left side click on Applications. You will see a list of applications that are loaded on the JNIOR. Check the box next to Modbus Server. This will tell the JNIOR to run the application when the JNIOR boots up. You will need to reboot the JNIOR at this point to get the MODBUS Server to start.

To verify that the MODBUS server is running we can list the listening network sockets using the netstat command.

Lastly, you can change a few configuration parameters for the MODBUS server under the registry tab.

Port 502 is the default MODBUS port. Most likely this will NOT need to be changed. The Timeout setting is the number of milliseconds to wait for a MODBUS client to poll the server. If a request is not made within this amount of time then the JNIOR will close the connection. If any configuration needs to be changed it will probably be the Login setting. Not very many MODBUS clients implement a login. If they dont log in then you will see the following in the Modbus Server log…

Setting the Login key to false will allow this device to connect successfully.