Troubleshooting RS-232 Connectivity Issues

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Instructions for troubleshooting RS-232 serial communication problems.

Many LI-COR gas analyzers feature RS-232 communication ports and sometimes these are the only communication channel available between the gas analyzer and a computer. Many new computers don’t have built in serial ports and USB to serial adapters are needed to facilitate this communication process. Communication problems between gas analyzer and windows interface software can happen due to issues with the USB to serial adapter or due to the lack of compatibility between the instrument firmware version and the windows interface software version. Terminal software such as Tera Term can be used troubleshoot such serial communication issues. Tera Term is an open source software developed under the BSD License. This software can be downloaded freely from the project web page at:

Tera Term can be used to verify instrument communication status or even check the instrument boot up sequence. The LI-8XXX series of analyzers that includes the LI-8100A, LI-840A and LI-820 can be diagnosed for Windows software communication issues using Tera Term. Opening Tera Term provides you with the option to connect through TCP/IP and through Serial. Selecting the serial option will display the different serial ports available in your computer. Once the appropriate port is selected, press OK to establish communication. To ensure that proper communication is possible, the serial port communication settings have to be verified and adjusted.

Common communication settings for the LI-8XXX analyzers are given below:

Analyzer Baud Rate Data Parity Stop Flow Control
LI-840/A 9600 8 none 1 none
LI-820 9600 8 none 1 none
LI-8100 (Firmware 3.0.1 and below) 57600 8 none 1 none
LI-8100A (Firmware 4.0.0 and above 38400 8 none 1 none

Connecting with the appropriate settings will show the data coming out of the terminal. This can indicate that the instrument is transmitting the data and the transmitted data is received by the computer. If proper baud rate settings are not selected, data may not appear, or extraneous data will appear on the terminal.

In order to ensure proper serial communications, it is essential to test if data can be transmitted from the computer to the instrument. If the transmit function is not working, the Windows interface software may not be able to communicate with the instrument. This may be due to incompatibility between the Windows software version and the embedded software currently loaded on the instrument. A program is provided (see below) to query the instrument for its embedded version to ensure the correct windows software is running.

Alternatively, commands can be sent to the gas analyzer through the Tera Term terminal. The easiest way to send a command through Tera Term is to type the command in a text editor like Notepad, copy it and use the Edit menu option paste<CR> to send the command to the instrument. The keyboard shortcut for this is Alt+R, which will work once the command is copied and after the active terminal window is clicked. Since the commands require a line feed at the end, it is essential to select the option for adding a linefeed for the transmitted commands in the terminal setup menu.

For example, the command to find the firmware version of the LI-8100 is


The command to find the firmware version of the LI-840(A) is


A Windows executable program is provided as a tool for detecting the firmware versions of the LI-8XXX series of analyzers. This executable uses similar communication protocols as the Tera Term software. Download it here:

A screen shot of the program is provided below:

The program detects the serial ports in your computer and displays it as a list where you can select the appropriate port. Buttons are provided to connect to the device and then to detect the firmware version. The option to stop the pump is also provided for the LI-8100(A). To operate this program start the executable, select the port to which the analyzer is connected and connect to the appropriate instrument. After the connection is established (the Connect button will change its status to “connected”), clicking the detect button will display the firmware version. A Reset button is provided to reset all the selections. Make sure the instrument is fully booted up and ready (ready light on) before connecting to the device. When the firmware is successfully detected the DETECT LI-8100 or 820/840 VER button will change to “DETECTED”. If you don’t get an output from the program you can press the RESET button to reset everything and try again.