General Measure Technology Co. Ltd. (China) - Guide to establishing communication between the GMT-X4 and the Modbus Poll software via the RS485 serial interface.

This tutorial walks you through the process of connecting the GMT-X4 to Modbus Poll using the RS485 serial port.

GMT-X4 Communication Interfaces

The GMT-X4 is equipped with several communication options:

• Serial ports (RS232 / RS485)
• CAN bus
• Ethernet (Modbus TCP/IP)
• Fieldbus systems: PROFINET, EtherCAT, Ethernet/IP and CC-Link IE

For this demonstration, the RS485 interface is used. The configuration steps for RS232 are nearly identical.

Hardware Setup

Connect the GMT-X4 device to the computer using the RS485 communication interface.

COM-1 Communication Configuration

From the GMT-X4 home screen, press Enter to access the Calibration Menu. Then press the Right Arrow key to open the Communication Parameter Menu.

1.1 Slave ID

• Select COM1 and confirm with Enter.
• Choose menu item 1.1 Slave ID.
• Adjust the ID value using the Up/Down keys.
• Press Enter to save.

Note: Default Slave ID is 01, valid range is 01–99. The Slave ID must match the one set in the host computer.

1.2 Baud Rate

• Navigate to 1.2 Baud Rate.
• Select the desired baud rate with Up/Down keys.
• Confirm the selection.

Note: The default baud rate is 38400. Both the instrument and Modbus Poll must use the same value.

1.3 Communication Protocol

• Choose 1.3 Communication Protocol.
• Select Modbus RTU.
• Confirm with Enter.

Note: Since the example uses Modbus Poll with the Modbus RTU protocol, Modbus RTU must be chosen here. Other protocols may be selected as required.

1.4 Data Format

• Open 1.4 Data Format.
• Keep the default value 8-E-1.
• Press Enter to confirm.

Note: The data format must be the same on both the GMT-X4 and the PC software.

1.5 Double Word Mode

• Select 1.5 Double Word Mode.
• Use the default mode AB-CD.
• Confirm the setting.

1.6 Continuous Transmission Interval

Because Modbus RTU is not a continuous transmission protocol, this setting is not required for this demonstration.

Note: This interval is used to prevent data overlap when transmission speed is too high. Adjust only if necessary.

Modbus Poll Software Configuration

After finishing the GMT-X4 settings, proceed to configure the Modbus Poll software on your computer.

Verify the Serial Port

• Open Device Manager.
• Go to Ports (COM & LPT) and check which COM port is assigned to the RS485 adapter.

Set Up Read/Write Parameters

• In Modbus Poll, open Read/Write Definition.
• Enter the same Slave ID that was set in the GMT-X4.

Establishing the Connection

• Go to Connection → Connect.

In the connection window, ensure the settings match the GMT-X4 parameters:

• Connection Type: Serial Port
• Serial Port: your RS485 COM port
• Baud Rate: 38400
• Data Format: 8-E-1

Click OK to establish the connection.

Once communication is successful:

• The GMT-X4 COM1 indicator will blink, confirming activity.
• The weight value displayed on the GMT-X4 will match the reading inside Modbus Poll.

This completes the setup for RS485 communication between the GMT-X4 and Modbus Poll.

Watch the demonstration video here: https://www.youtube.com/watch?v=BN1eEqpbBcs

The next video will cover Ethernet communication (Modbus TCP/IP) setup on the GMT-X4.