آموزش اتصال اینورتر به PLC با Modbus RTU | راهنمای کاربردی

Connecting an inverter (VFD) to a PLC using Modbus RTU is an effective way to control, monitor, and automate electric motors in industrial applications. By establishing communication between a Programmable Logic Controller (PLC) and a Variable Frequency Drive (VFD), operators can send control commands, adjust motor speed, and retrieve operating data without relying on separate wiring for every control and feedback signal.

One of the most widely used methods for PLC-to-VFD communication is Modbus RTU over RS-485. This serial communication method allows industrial controllers to exchange information with compatible motor drives through a shared communication network.

In this guide, we explain how to connect an inverter to a PLC, configure RS-485 communication, set Modbus RTU parameters, read and write registers, and troubleshoot common communication problems.

The practical examples are based on the XIMA G Series industrial VFD, using the manufacturer’s official installation manual, Version V5.2 (software version H3.24).

Safety Notice: This guide is intended for qualified personnel familiar with industrial electrical systems and automation. Wiring, commissioning, and motor operation must follow the manufacturer’s instructions and applicable safety requirements. Software stop commands and Modbus communication failures must never be considered substitutes for independent emergency-stop systems or appropriate machine safety circuits.

What Is PLC-to-Inverter Communication?

A Variable Frequency Drive (VFD), also known as a motor inverter, controls the operating speed of a compatible AC motor by adjusting its output frequency and voltage.

A Programmable Logic Controller (PLC) is an industrial controller that processes input signals and executes programmed instructions to control machinery and industrial processes.

When an inverter and a PLC communicate, the PLC can control selected drive functions and monitor operating conditions through the communication interface.

For example, in a conveyor system, the PLC can adjust the inverter’s frequency reference according to production requirements. At the same time, the VFD can transmit information such as output frequency, current, and operating status back to the controller.

Main Benefits of Connecting a VFD to a PLC

PLC-to-VFD communication supports several industrial automation functions:

• Sending start, stop, and direction commands when permitted by the application.
• Adjusting the motor’s frequency reference and operating speed.
• Reading output frequency, current, and voltage.
• Monitoring drive status and fault conditions.
• Coordinating multiple motor drives within one control system.
• Transferring operating information to an HMI or industrial monitoring platform.

The exact capabilities depend on the VFD model, firmware version, supported communication functions, and available registers.

Methods of Connecting an Inverter to a PLC

There are several ways to establish communication or control between a PLC and an inverter.

The appropriate method depends on the controller, drive capabilities, required control functions, number of devices, and overall system architecture.

1. Digital Input and Output Connection

In this method, the PLC’s digital outputs connect directly to the inverter’s digital control inputs.

For example, one output may provide the run command, while another may select motor direction or activate a different control function, depending on the VFD configuration.

Digital I/O connections are suitable for relatively simple control systems. However, the number of signals that can be exchanged is limited by the available hardware inputs and outputs.

2. Analog Signal Connection

Some industrial control systems use an analog output from the PLC to provide the inverter’s speed reference.

Depending on the equipment, a compatible analog signal can be used to adjust the frequency reference continuously.

This approach is useful for variable-speed applications, but obtaining detailed operating data from the drive may require additional communication channels.

3. Modbus RTU Communication

With Modbus RTU, the PLC and inverter exchange digital messages through a serial communication network.

Instead of dedicating individual wires to multiple commands and feedback signals, the PLC can send requests to read or write supported drive registers.

This allows multiple control and monitoring functions to be handled through a common communication interface.

Modbus communication does not necessarily eliminate the need for digital inputs, analog signals, or independent safety wiring. Many industrial systems combine these methods.

What Is Modbus RTU and How Does It Work?

Modbus RTU is an industrial serial communication protocol used to exchange information between compatible automation devices.

In a typical Modbus RTU network, one device operates as the Master, while the connected devices operate as Slaves.

Modern documentation may also use the terms Client and Server.

When connecting a PLC to an inverter:

• The PLC typically acts as the Modbus Master.
• The VFD operates as a Modbus Slave.
• The PLC sends requests to a specific device address.
• The inverter processes supported requests and returns the appropriate response.

How Modbus RTU Communication Works

A typical communication sequence follows these steps:

1. The PLC sends a read or write request to the inverter’s Slave address.
2. The inverter receives and checks the request.
3. If the command is valid and permitted, the requested operation is performed.
4. The inverter returns the relevant response when required by the protocol.
5. The PLC processes the received information.

Modbus RTU messages contain fields such as the device address, Function Code, associated data, and a Cyclic Redundancy Check (CRC) for communication error detection.

Modbus RTU vs. RS-485: What Is the Difference?

Although Modbus RTU and RS-485 are frequently mentioned together, they are not the same technology.

RS-485 defines electrical characteristics for serial data transmission. It provides the physical communication interface used by compatible industrial devices.

Modbus RTU is a communication protocol that defines how messages are organized, transmitted, and interpreted.

In simple terms:

• RS-485 provides the electrical communication medium.
• Modbus RTU defines the structure and meaning of exchanged messages.

An RS-485 interface does not automatically guarantee Modbus RTU support. The supported protocol must be confirmed in the equipment documentation.

Equipment Required for PLC-to-VFD Modbus RTU Communication

Before starting the installation, make sure both devices support a compatible communication method.

The typical requirements include:

• A VFD equipped with an RS-485 interface and documented Modbus RTU support.
• A PLC with an RS-485 communication port or compatible communication module.
• A suitable industrial communication cable.
• Appropriate shielding and termination arrangements.
• The technical manuals for both the PLC and VFD.
• PLC programming software for configuring, testing, and monitoring communication.

If the PLC only has an Ethernet interface, it cannot be directly wired to the inverter’s RS-485 terminals.

In that case, a compatible communication module or gateway is required, and protocol compatibility must also be verified.

Before wiring, identify the exact device models, communication terminal labels, reference connections, and grounding requirements.

How to Wire RS-485 Between a PLC and an Inverter

RS-485 communication is commonly implemented using a two-wire differential data pair.

The two signal lines may be identified differently by equipment manufacturers. Some use A and B, while others use positive and negative signal designations.

According to the official XIMA G Series VFD documentation, the RS-485 communication terminals are identified as:

• A(S+)
• B(S−)

The wiring must follow the manufacturer’s terminal diagram and verified signal polarity at both devices.

Connecting One PLC to One VFD

For a basic point-to-point connection, the PLC’s RS-485 communication terminals are connected to the corresponding communication terminals on the inverter.

However, terminal names should not be matched solely by the letters A and B.

RS-485 terminal naming conventions can vary between manufacturers. Always confirm the electrical signal definitions in the manuals for both devices.

Important installation considerations include:

• Use communication cable suitable for the industrial environment.
• Route communication cables appropriately.
• Avoid unsuitable routing near high-noise power circuits.
• Follow the manufacturer’s instructions for signal common, grounding, and shielding.
• Select and install line termination correctly.
• Verify the wiring before applying control commands.

For initial testing, establish communication between one PLC and one drive under controlled conditions before expanding the network.

Connecting Multiple VFDs to One PLC

Modbus RTU also supports communication with multiple compatible drives on a shared network.

In a multi-drive installation, every inverter must have a unique Slave address.

The PLC can then direct read and write requests to the required device.

The XIMA G Series manual describes communication between a Master and multiple XIMA drives, including an arrangement with up to 15 devices connected to one Master.

The actual supported network configuration must be checked against the relevant device models, communication interface limitations, cable length, electrical conditions, and manufacturer specifications.

RS-485 Termination Resistors

Correct termination helps minimize signal reflections and improve communication reliability.

The XIMA G Series V5.2 manual specifies a 150-ohm termination resistor in its RS-485 communication arrangement, particularly for longer communication lines.

This value is associated with the referenced XIMA documentation and should not be interpreted as a universal requirement for every RS-485 network.

When designing the network, check:

• Termination at the ends of the communication bus.
• Any internal termination already present in connected devices.
• The required line-biasing arrangement.
• The electrical characteristics and installation conditions of the network.

Incorrect or excessive termination can reduce communication reliability.

Configuring Modbus RTU Parameters on a XIMA G Series VFD

For successful communication, the inverter and PLC must use compatible serial communication settings.

The XIMA G Series V5.2 manual documents the relevant Modbus parameters in its system parameter section.

Main Modbus RTU Parameters

These parameters correspond to the referenced manual version. They must not be applied to other XIMA drive models or firmware versions without checking their respective documentation.

1. Setting the Baud Rate

The Baud Rate determines the serial data transmission speed.

The XIMA G Series manual lists the following settings:

The documented default value is 38400 bps, corresponding to parameter value 3.

The PLC and VFD must be configured with matching Baud Rates.

The appropriate communication speed depends on cable length, network quality, electromagnetic interference, and hardware capabilities.

Higher Baud Rates do not automatically provide greater communication reliability.

2. Setting the Modbus Slave Address

The Serial Address identifies an individual inverter on the Modbus network.

If two drives use the same Slave address, communication conflicts may occur.

Each drive should therefore have a unique address, and the PLC must use that same address when sending requests.

In the XIMA G Series V5.2 manual, the serial address parameter table specifies a range of 1 to 240. However, the communication section also refers to a range of 1 to 200.

Because these ranges differ within the documentation, the applicable limit should be confirmed for the installed firmware and hardware version.

Modbus address 0 is reserved for broadcast messages. Devices receiving a broadcast request do not return normal individual responses.

Broadcast commands therefore require careful consideration in motor-control applications.

3. Configuring Parity and Stop Bits

Parity is another essential serial communication setting.

The XIMA G Series manual defines three options for register 0x031D:

According to the V5.2 communication specifications, the serial frame uses one start bit and eight data bits.

When parity is disabled, two stop bits are used.

When Odd or Even parity is selected, one stop bit is used.

For example, 8N2 means:

• 8 data bits.
• No parity.
• 2 stop bits.

With the documented default settings, the serial configuration is 38400 bps, no parity, and two stop bits.

The PLC must use matching data-bit, parity, and stop-bit settings.

4. Configuring Communication Timeout Behavior

Communication between a PLC and an inverter may be interrupted by a damaged cable, electromagnetic interference, or failure of the communication program.

On the XIMA G Series VFD, register 0x031E defines the Communication Timeout period.

Register 0x031F determines the drive’s response when the communication timeout condition occurs.

The documented options include:

• Taking no additional action.
• Stopping the drive according to its configured stopping behavior.
• Disabling the drive output.

The correct response depends on the application and machine risk assessment.

For example, allowing a motor to coast to a stop may be acceptable in one application but hazardous in another.

Important: Communication timeout detection is not a certified functional safety mechanism and must not replace independent emergency-stop or machine-protection functions.

Selecting Modbus as the Command and Frequency Source

Configuring the serial communication parameters alone is not sufficient to control motor operation.

The inverter must also be configured to accept the appropriate commands and frequency reference from the Modbus interface.

The XIMA G Series provides separate settings for these functions.

Selecting Modbus for Run and Stop Commands

The Digital Input Configuration parameter determines the source of certain drive control commands.

In the referenced XIMA G Series manual:

• Parameter: io01
• Register Address: 0x0200
• Modbus Control Setting: 16

When this setting is selected, the relevant control commands can be received through the Modbus communication interface.

Before modifying the parameter, inspect the existing control circuit, safety conditions, and the effects of switching the command source.

Selecting Modbus as the Frequency Reference

The Analog Input Configuration parameter determines the source of the drive’s frequency reference or associated control quantity.

For the referenced XIMA G Series version:

• Parameter: io05
• Register Address: 0x0204
• Modbus Frequency Reference Setting: 6

Receiving run commands through Modbus and receiving the frequency reference through Modbus are two separate functions.

It is possible for the PLC to communicate with the inverter successfully while the drive continues to use a different frequency reference source.

In that situation, writing a new frequency value from the PLC may not change the motor’s operating speed.

Understanding XIMA VFD Modbus Registers

Modbus communication uses register addresses to exchange control and monitoring data.

Depending on the manufacturer and register definition, an address may represent a drive parameter, an operating value, a status word, or a control command.

The XIMA G Series manual documents the following Function Codes for the relevant operations:

• Function Code 0x03: Read supported registers.
• Function Code 0x06: Write a single register.

Support for these functions does not automatically mean the drive supports every Modbus Function Code.

Writable Modbus Registers

The principal writable registers documented for the XIMA G Series include:

The control register 0x2000 contains individual bits assigned to functions such as Enable, Start, direction selection, and Jog.

For example, the manual identifies:

• Bit 0: Enable
• Bit 1: Start

The complete control word must be constructed according to the drive state, other relevant bits, and the machine’s control logic.

Writing a fixed control word without verifying the permitted operating conditions is not an appropriate method of commissioning machinery.

Readable Modbus Registers

The XIMA G Series also provides registers for monitoring drive operation.

These values can be used in the PLC program for monitoring, data logging, and displaying information on an HMI.

For example, the PLC can read the output frequency register and convert its raw value into hertz using the documented scaling factor.

Important Note About Modbus Register Addressing

The XIMA manual presents register addresses in hexadecimal format.

However, PLC software packages do not always use the same register-addressing conventions.

Some require the raw Modbus address, while others use Holding Register numbering or internal offsets.

For this reason, the address displayed in the PLC programming environment may differ from the address printed in the drive manual.

Before sending commands, verify the addressing convention used by the specific PLC communication library or Modbus module.

Practical Example: Reading VFD Output Frequency with a PLC

Consider a XIMA G Series inverter connected to a PLC through Modbus RTU.

The objective is to read the actual output frequency and display it on an HMI.

This example assumes that serial communication has already been configured and the equipment is in a safe operating condition.

Step 1: Match the Communication Settings

First, ensure that the PLC and VFD use compatible communication settings.

For demonstration purposes, a Baud Rate of 9600 bps may be selected.

In the XIMA G Series documentation, this corresponds to parameter value 1 at register 0x031B.

Next, configure the Slave address, Parity, and Stop Bits consistently on both devices.

These are example settings, not a universal recommendation for all installations.

Step 2: Send a Register Read Request

To read the inverter’s output frequency, the PLC creates a Modbus request with the following information:

The PLC software or Modbus communication library must generate the appropriate RTU message and handle CRC calculation correctly.

Step 3: Convert the Received Value

According to the XIMA G Series documentation, the output frequency value uses a scaling factor of 10.

For example, suppose the PLC reads the integer value 350 from register 0x2101.

The actual output frequency is calculated as:

350 ÷ 10 = 35.0 Hz

The PLC program should convert the raw register value before displaying it as an engineering unit.

Other registers may use different scaling factors.

For example, the referenced manual indicates a one-decimal-place representation for output current, while the output voltage is represented without decimal places.

Step 4: Display the Frequency on the HMI

After converting the value, the PLC can store it in a suitable variable and display it on an HMI.

The HMI should also indicate communication validity.

If the network disconnects or the received data becomes invalid, the last recorded frequency should not continue to appear as a current live measurement without an appropriate warning.

Practical Example: Setting VFD Frequency Through a PLC

Another important application of Modbus RTU is sending the frequency reference from a PLC to an inverter.

According to the official XIMA G Series documentation, register 0x2001 is used for the frequency reference.

The value must be transmitted using the scaling specified by the manufacturer.

For example, a frequency reference of 50.0 Hz is represented by the integer value 500.

The conceptual Modbus request is:

This example illustrates register scaling only.

A 50 Hz reference is not an appropriate universal operating setting for every motor or application.

Before sending an actual control command, verify that:

• The motor and inverter are correctly selected for the application.
• Frequency limits and acceleration settings are appropriate.
• Modbus is selected as the drive’s frequency reference source.
• The run and stop control logic has been designed safely.
• The PLC monitors relevant operating conditions and drive faults.

Important: Writing a frequency reference is different from sending a Start command.

A successful Modbus write response also does not prove that the motor has reached the requested frequency.

Actual operation must be verified through the appropriate monitoring registers.

Difference Between VFD Commands and Actual Operating Status

A common mistake in PLC-to-VFD communication is treating a control command as confirmation of actual drive operation.

For example, the PLC may send a 40 Hz frequency reference while the motor is still accelerating.

During this period, the requested frequency and actual output frequency are different.

For reliable monitoring, three types of information should be distinguished:

1. Command value: The control value transmitted by the PLC.
2. Actual value: The operating value read from the inverter’s monitoring registers.
3. Communication and drive status: Information indicating whether the response is valid and what state the inverter is currently in.

This distinction is particularly important when VFD feedback is used for subsequent control decisions.

Common PLC-to-VFD Modbus RTU Problems and Troubleshooting

Modbus RTU communication may fail during initial commissioning or become unstable after the system has been operating.

A structured troubleshooting process helps determine whether the problem involves wiring, communication settings, register addressing, message structure, or drive control logic.

1. The PLC Receives No Response from the VFD

If all Modbus requests time out, begin by checking the physical communication interface and serial settings.

Verify the following:

• Correct RS-485 terminal connections.
• Matching Baud Rates.
• Matching Parity and Stop Bit settings.
• Correct Modbus Slave address.
• Active PLC communication port and Master program.
• Suitable communication cable and termination.

Start troubleshooting with a simple read request to a supported monitoring register before sending motor-control commands.

2. Modbus Communication Works, but the Frequency Does Not Change

In this situation, the PLC may be able to read inverter data while the frequency reference is still assigned to another input source.

For the XIMA G Series, check the Analog Input Configuration parameter and confirm that the Modbus frequency reference option is selected.

Also verify the register address, Function Code, and transmitted data value.

3. The Start Command Is Sent, but the Motor Does Not Run

Receiving a Modbus command does not automatically mean that the motor is permitted to start.

In addition to checking the command source, verify:

• Drive Enable status.
• Active fault conditions.
• Relevant drive control limitations.
• Permissive conditions required for motor operation.

Register 0x2100 contains command and operating status information for the referenced XIMA G Series.

These values can assist troubleshooting, but independent machine safety conditions must still be evaluated through the appropriate safety systems.

4. RS-485 Communication Disconnects Intermittently

Intermittent communication problems may be caused by several factors:

• Electromagnetic interference.
• Damaged or unsuitable communication cable.
• Poor cable routing.
• Incorrect termination or line biasing.
• Inappropriate PLC response timeout settings.
• Excessive request frequency relative to network capacity.
• Signal reference or common-mode voltage problems.

Make changes systematically and test the effect of each correction.

5. The VFD Returns a Modbus Exception Response

The XIMA G Series documentation describes Modbus exception conditions including the following:

For example, attempting to write to a read-only register may result in an exception response.

The PLC program should distinguish exception responses from communication timeouts.

An exception response means that the drive received the message but could not accept or execute the requested operation.

Best Practices for Reliable PLC-to-VFD Modbus RTU Communication

Establishing the first successful communication response is only the beginning.

The system must also operate reliably under normal industrial conditions.

Test Communication Before Enabling Motor Commands

Begin by reading a supported monitoring register.

This helps identify serial communication and addressing issues without issuing motion commands.

Handle Communication Timeouts and Errors

The PLC program should distinguish between:

• Valid responses.
• Delayed responses.
• Modbus exception responses.
• Complete loss of communication.

Retry limits and request intervals should be selected according to the communication network’s capabilities.

Avoid Duplicate Slave Addresses

Each drive on a shared Modbus network must have a unique address.

Recording Slave addresses in the electrical drawings and project documentation makes future troubleshooting and system expansion easier.

Keep Safety Functions Independent

Modbus RTU is intended for control and monitoring data exchange.

It is not inherently a functional safety system.

For machinery where unexpected movement creates a hazard, emergency-stop functions, personnel protection, and prevention of unexpected startup must be designed according to the application risk assessment and applicable safety requirements.

Match the Manual to the Installed VFD

Different XIMA VFD models or firmware versions may use different communication parameters and registers.

Always confirm the exact inverter model and documentation version before changing settings.

Modbus RTU Communication on the XIMA G Series Industrial VFD

The XIMA G Series industrial VFD supports RS-485 serial communication using the Modbus RTU protocol, as documented in its official installation manual.

The communication interface supports control commands and access to operating data through documented registers.

Depending on the supported functions and firmware version, available information includes:

• Drive command and operating status.
• Output frequency.
• Output current and voltage.
• Heatsink temperature.
• Output power.
• Terminal status.

Communication settings such as Baud Rate, Slave address, Parity, and timeout behavior are also documented in the system parameters.

For technical reference, consult the official XIMA G Series Installation Manual.

For general information about the manufacturer and its industrial drive solutions, visit the XIMA website.

Frequently Asked Questions About Connecting a VFD to a PLC

Can Any Inverter Be Connected to a PLC Using Modbus RTU?

No. The inverter must support Modbus RTU and provide a compatible communication interface.

An RS-485 port alone does not guarantee Modbus RTU compatibility.

Always verify the supported protocol in the manufacturer’s documentation.

Is a Separate Communication Module Required to Connect a VFD to a PLC?

Not always.

If the PLC already includes a compatible RS-485 port and can operate as a Modbus RTU Master, an additional module may not be required.

Otherwise, a suitable communication module or gateway must be selected according to the PLC’s hardware and supported protocols.

Why Does the PLC Communicate with the VFD but Fail to Start the Motor?

Possible causes include an incorrect command source, missing Enable conditions, active inverter faults, or application-specific control restrictions.

Check the drive’s command source configuration, relevant control registers, and the PLC’s control logic.

What Is the Difference Between Modbus RTU and Modbus TCP?

Modbus RTU is commonly used for serial communication, including networks based on RS-485.

Modbus TCP is designed for Modbus communication over TCP/IP networks.

The communication media and message transport differ, so connecting these systems may require a suitable gateway.

Can a PLC Change the Frequency of a XIMA Inverter Through Modbus RTU?

Yes, when supported by the drive model and correctly configured.

For the referenced XIMA G Series VFD, the frequency reference can be written through Modbus using register 0x2001.

The corresponding frequency source must also be configured to accept the reference from Modbus.

Will the Inverter Automatically Stop if Communication with the PLC Is Lost?

Not necessarily.

The behavior depends on the drive settings.

The XIMA G Series includes parameters for communication timeout detection and the response to communication loss.

The appropriate behavior must be selected based on the process requirements and machine safety assessment.

Conclusion: Connecting a VFD to a PLC Through Modbus RTU

Connecting a VFD to a PLC using Modbus RTU provides a practical method for exchanging control commands and operating information in industrial automation systems.

With a properly configured communication interface, the PLC can adjust the inverter’s frequency reference, read operating status, and use drive feedback for monitoring and process control.

Successful implementation depends on three essential factors:

1. Correct RS-485 wiring and network design.
2. Matching serial communication parameters.
3. Accurate use of the manufacturer’s documented Modbus registers.

For the XIMA G Series industrial inverter, the required communication settings, register addresses, and control functions are documented in the official installation manual.

Using these specifications alongside a properly designed PLC program helps create a more reliable commissioning, troubleshooting, and maintenance process.

To learn more about industrial motor control solutions, explore the XIMA official website and consult the XIMA G Series technical documentation before configuring your equipment.