Modbus RTU RS-485 Setup


Welcome. This video will demonstrate setup and basic procedures for communicating with your FUSION controller using the Modbus RTU fieldbus over an RS-485 serial interface.

Modbus RTU is a fieldbus that allows digital communication between a PLC and the controller.

There are some advantages to using Modbus RTU over an analog interface:


  • Allows access to all user parameters
  • Eliminates analog error
  • Lowers your installation costs


Before we begin, you will need to install the Control Panel software, which is provided free with your controller. Connect power to the controller and connect to your computer with a USB cable. Now you will need to set up an interface between your factory controller, or PLC, and the power controller. For this tutorial, we will use an RS-485 serial port, connected to a laptop on Comm 3.

Let's look at a few basic procedures. First, we will set up the slave address and communications settings on our controller.

Open the Control Panel software. On the lefthand side of the screen, you should see "USB" selected as the interface. Click the "Connect" button.

Next choose the Digital Communication tab.  You'll see "Common Settings" – we'll come back to that later – and below that, "RS-485 Communication." For this demonstration, we'll enter "1" for our Modbus address, or Slave address, and choose Modbus standard settings: 19200 for the Baud Rate, and E_8_1 for the Byte Format. 

Now let's connect to the controller using the Modbus RTU interface. Select that option here and click "Connect." Your available comm ports will be based on your own hardware configuration; for our tutorial we're using an RS-485 port on Comm 3. We match the settings to what we set up before: Slave Address is 1, 19200, and E_8_1. 

Ok, we can see the Transmit/Receive lights are blinking, so we're getting communication.

Next we will discuss reading and writing parameters. 

We'll specifically look at two kinds of parameters: setup parameters, and monitor parameters.

Setup parameters are numbered 1 - 199, and can both be read and written to. For example - digital setpoint, firing modes, current limits ... these are all setup parameters.

Parameters 200 through 389 are Monitor parameters. These are read-only; they consist of process data you may want to monitor, like load voltage, or load current, or load power.

Ok, let's look at a very typical scenario: First, say we want to write a digital setpoint, then, read a monitor parameter, like Load Current A on phase 3, and finally process the information in order to update the setpoint. And then we'll repeat the process.

Writing the setpoint is accomplished with Modbus Function 6, or "Write Single Register" which requires the following information: Slave ID 1, Parameter # 100, and as a value, 50 % or 5000.

To read monitor parameters for Load Current A, we use Modbus Function 3, "Read Holding Registers." Enter 1 for the Slave ID, 222 for the parameter, and 1 for bytes. We recommend consulting your PLC documentation for specific instructions on how to implement Modbus for your particular controller.

You can also find modbus specifications online at, under "Technical Resources."

The FUSION Control Panel program also has a nice feature for finding parameters. Just hover your cursor over the value or setting you're looking for, and the corresponding parameter number will appear under the cursor.

We will briefly talk about overloading. If you are reading or writing too many parameters at a time, the controller may become overloaded. To avoid overloading, use block reads to collect data more efficiently. A block read consists of up to 16 parameters. Furthermore, do not poll the controller more than four block reads per second.

Now let's address Digital Communication Failure. Under the digital communication tab on the Control Panel we see "Common Settings." If communication is interrupted, whether because of a cabling issue or your PLC has shut down, this is where you specify how your controller will react. It's known default state that you've defined.

The Comm Heartbeat is like a watchdog for your controller. The default factory setting is 0, which stands for an infinite timeout. When it is set to zero, if there is a communication failure, the controller will continue running as it has been based on your last command. But maybe you want to know, or take a specific action when you have a communication failure.

You have two options, indicated by these two radial buttons: continue, and stop. For our demonstration, our timeout will be set for 5 seconds. This means the controller expects a read or write from the fieldbus, at least every 5 seconds, and if communication has been interrupted, it does one of two things:

If you have the controller set to continue, it continues running at your last digital command until network communication is restored. We can demonstrate this by removing the cable. After 5 seconds, the controller displays a warning and holds the last command. So to restore communication, we correct the failure and make sure the factory controller or the PLC is communicating with the controller. Then the warning alarm will disappear.

If you select "stop" and communication is interrupted, the controller goes into an inhibit state and stops operating. This is a fault state, indicated on the control panel here. You may choose this option as a safety precaution, it all depends on your specific application and preferences.

Stop mode requires you to do two things to get the controller back to run state: First, restore digital communication - fix the problem, in our case restore the cable, and make sure there is communication between the factory controller or PLC and the controller. Then cycle run/reset on P1 to clear the fault. These two steps need to happen in sequence; first restore digital communication, and second cycle run/reset.

An additional option would be to have the controller notify an external signaling device, like a light tower or alarm system. You can find this option under the "System" tab on the control panel; choose "Communication Error" here to trigger Relay 1 or Relay 2 in the event of a communication failure.

For more detailed instructions, visit our website at or contact the experts at Control Concepts Inc.