Archived:Troubleshooting Serial Communication in Linux

Updated Apr 25, 2023

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Operating System

  • Linux


Red Hat version 6.2 or later

I am using NI-VISA to communicate serially with my device, but I keep getting an error at the VISA Open VI in LabVIEW. I have a Linux operating system (Red Hat version 6.2 or later). What troubleshooting steps can I take to make sure my system is configured properly?

Note: This document only applies to versions of NI software released prior to NI Linux Device Drivers. For new designs, NI recommends using NI Linux Device Drivers to take advantage of repository-based installation and more frequent updates to support.

Verify the following configuration settings in the process of troubleshooting your system:
  1. Do you have the NI-VISA driver installed?

    You must have the NI-VISA driver installed in addition to the driver specific to your device (that is, the NI-488.2 driver if you are using a National Instruments GPIB controller) for the serial communication to work.
  2. Is the NI-VISA driver installed in the right place?

    /usr/local/vxipnp... is the default path to the home directory of NI-VISA. Environment variable VXIPNPPATH should point to this path or the path you specified when you perform an env query. 
  3. Is there a correct series of links that point to the hard file in the user library?

    Search the path for the file by performing the following query: ld_library_path The user library should include a soft link for that points to another soft link. The second soft link points to the hard file of If this path is not correct, find the hard file of and create a link that points to it in the user library.

    If the ld_library_path query does not work on your computer, locate the file using the find / -name '' -print command or locate Once you locate this file, create a link to it in the user library.
  4. Do you have permission to read and write to your serial port?

    By default, only the administrator is allowed serial port access. Log on as an administrator (or root). Make sure that users have read/write access to port 0 by performing the following query: ls -l /dev/ttyS0. You must perform the same query for all ports that you want to read or write to (for example, /dev/ttyS0 for port 1, /dev/ttyS1 for port 2, and so on). Alternatively, you can query all ports with ls -l /dev/ttyS*. You can add permission for everyone to read and write from your serial ports by issuing chmod 666 /dev/ttyS* The command allows everyone to read and write to all serial ports. 
  5. Do you have permission to read the /usr/local/natinst and /usr/local/vxipnpdirectories?

    LabVIEW and the VISA driver must be able to access files in these directories. It is common to install software as root and then use it as another user. If your user permissions are sufficiently restricted, you may not be able to use VISA at all.
  6. Are you using the correct VISA Resource Name?

    When you refer to serial ports in Linux, you should use only ASRL1::INSTRASRL2::INSTR2, and so on. COM1COM2, or ttyS0 and ttyS1 are not correct.
  7. Are your serial ports enabled?

    Run the visaconf tool to query your resources and display all serial ports. Make sure all ports you want to use are listed as enabled by double-clicking on the resource and viewing the resource window. If a port is disabled, change it to the enabled state.

    - What if I am unable to run the visaconf tool?

    This command will not work unless /usr/local/vxipnp/Linux/NIvisa is in the users PATH. If the PATH does not exist, you must use the /usr/local/vxipnp/Linux/NIvisa/visaconf command instead.

    - What if my serial port is listed as busy?

    Try creating a new VI in LabVIEW that uses the VISA Clear VI. Set the VISA resource name to the serial port that was listed as busy (that is, ASRL2::INSTR for port 2, ASRL3::INSTR for port 3, and so on) and run the VI a few times to clear the port. If the serial port is still busy, make sure other applications on your computer are not using it. 

    - What if none of my serial ports are listed?

    You want to add static and then manually add the serial ports (/dev/ttyS0/dev/ttyS1, and so on).
  8. Is your external cabling correct?

    Verify that your instrument is connected to port 1, if that is the port you want to use.
  9. Is your software program correct?

    One way to determine whether your error is caused by improperly configured serial ports or by your software program is to remove the software. Use NIvisaic for Linux to scan all ports and find all resources. You also can write to one port and have another read from it to test its functionality. If the ports behave correctly, the problem lies within your LabVIEW program. NIvisaic is similar to Measurement & Automation Explorer, which is available in Windows. The NIvisaic command works only if the PATH is included (see part 6 above).

    Another good way to test your serial ports is to use one of the LabVIEW examples, such as the Easy Loopback VI. You can search the Example Program Database (linked below) or use one of the LabVIEW shipping examples by selecting Help»Find Examples and typing serial in the Search tab.