Problems with USB to RS485 converter

I have my T3-TB running with sensors and can see the input values with the T3000 software when connected via ethernet.

Now, I want to be able to read the input registers over Modbus rtu on the main 485 port.

I have tried setting up the main rs485 port and reading it with the Modbus tool. However, it appears the Modbus is actually coming via the tcp link. If I disconnect the ethernet, I can’t read the registers.

  1. Can the ethernet and '485 Modbus port run simultaneously?
  2. I have set the T3 to be a Modbus slave, ID=255, baud 19200. For testing, register 7484 has a temperature sensor which shows up correctly on the frontend display over ethernet.
  3. Using the Modbus tool, the Rx light blinks on the T3-TB module, but no data is returned.

When the system is finally deployed, the T3 will be remote in the field , near the sensors. I will have a wired '485 link back to the main plc and it will poll the T3 via Modbus to get the sensor values.

Can you point me to a tutorial or example to help me with this problem?


The RS485 ports can be set up as Modbus and you can connect with T3000 to manage the device well, including Modbus tool to read & write registers as well as the full GUI interface for all the various parameters. Please disconnect the Ethernet cable, then scan with only the RS48 connection active. T3000 should discover the device and you can manage it normally, including polling of Modbus registers as you describe.


Thanks Maurice

It was mainly an issue with a USB to 485 converter which had reversed polarity.

All is well now!