You might face the following issues while using Modbus I/O servers. Follow the guidelines to troubleshoot Modbus I/O servers.

  • Modbus generates the following error after you deploy a Modbus Master I/O server. Modbus failure: LabVIEW DSC error code: -1967353900. The Modbus I/O server failed to allocate a serial port.
    • Check the serial port you configured for the I/O server. Ensure that no other program occupies the serial port.
    • The Shared Variable Engine (SVE) registers available serial ports only when the SVE is initializing. Ensure that no process or application occupies any serial ports when the SVE is initializing. The SVE does not register occupied serial ports. If you install new serial ports using the ENET232, USB, or Virtual Ports when the SVE is running, restart the computer or the SVE to register the new serial ports.
  • Modbus generates the following error after you deploy a Modbus Slave I/O server. Modbus failure: LabVIEW DSC error code: -1967353912. The Modbus Slave I/O server failed to bind to port 502.
    • Check whether you deployed duplicate Modbus Slave (Ethernet) I/O servers. Modbus Slave (Ethernet) I/O servers listen on port 502 to communicate with Modbus Master I/O servers. Duplicate Modbus Slave (Ethernet) I/O servers cause errors when these I/O servers listen on port 502.
  • Modbus cannot read/write array types.
    • Ensure that the length of the array does not exceed the maximum length of arrays that you specified. You can specify the maximum length of arrays in the Maximum Data Points Per Command section of the Advanced Attribute Settings dialog box.
  • Modbus generates errors when the Modbus Master I/O server communicates with Modbus Slave I/O servers.
    • Assign a unique address to each Modbus Slave (Serial) I/O server. If multiple Modbus Slave (Serial) I/O servers have the same addresses, the Modbus Master I/O server communicates with only the first Modbus Slave I/O server that the SVE deploys.
  • Modbus does not work at low baud rates.
    • Increase the Timeout delay value in the Configure Modbus I/O Server dialog box. For example, to use the modbus I/O server at baud rate 2400, set Timeout delay to 500 ms or greater.

Understanding Modbus Communication in LabVIEW