Configuring the LI-350 (SDI-12)

This section describes how to apply new settings on the LI-350 using a terminal program and SDI-12-to-USB adapter.

Connecting the LI-350 to an SDI-12 data acquisition device

The LI-350 provides a digital output using the SDI-12 protocol. LI-350 sensors that feature the M12 connector are designed for use with some compatible LI-COR products. For these sensors, all functions are exposed in the device interface. Normally, you won't need to configure the connectorized LI-350 beyond applying some settings in the host device. Never-the-less, see Table 4‑1 for wire assignments.

LI-350 sensors that feature bare leads can be configured using a datalogger or an SDI-12-to-USB adapter.

Table 4‑1. Wire assignments for the LI-350.
Color Function M12 Connector Pin Assignments
Black Ground 1: Power 4: Ground
Red Power (4 to 15 VDC)    
White Data   3: Data
Bare Connects to ground      

SDI-12 translator tutorial

Figure 4‑1. SDI-12-to-USB translators allows communication with the soil moisture sensor.

The following items are required to complete this tutorial:

  • LI-350 soil moisture sensor (SDI-12; bare leads).

  • SDI-12-to-USB Translator (e.g., vegetronix.com translator or Dr. John Liu adapter liudresllc.com ).

  • Personal Computer (Windows 10/11).

  • Terminal Emulator (e.g., Tera Term).

You will connect the wires from the soil moisture sensor to the data and ground connectors on the translator. Depending on the translator, you may need to power the sensor from an external power supply (see Figure 4‑2).

Connecting to a computer with an SDI-12 translator

Connect the soil moisture sensor to the SDI-12 translator and power supply, and then connect the USB cable to your computer. The computer will establish a COM port, which may be indicated in the notifications or found in the Device Manager.

Soil probe connected to SDI-12 to USB adapter.Soil probe connected to SDI-12 to USB adapter.
Figure 4‑2. The Liu SDI-12 Adapter (top) can power the soil probe via the USB power. With the Vegetronix SDI-12 Translator (bottom), the soil probe must be powered externally. In either case, data from the probe can be read in a terminal program.

To connect to the SDI-12 translator:

  1. Launch a terminal program, such as Tera Term.

  2. Select Serial as the connection type, select the port for the USB translator, and then click OK.

In the default configuration, Tera Term will pass your keystrokes directly to the COM port - you will not see what you have typed. Change the setting under Setup > Terminal, and check Local Echo and your keystrokes will appear in the terminal, along with replies from the sensor. This is a matter of convenience - so you can see what you are sending to the device.

Note: The New-line Receive and Transmit settings must be CR.

Communicating with a command-line interface

This section presents a variety of commands and replies that you can use to configure and view data from the device. This is a practical tutorial; you won't do this when the LI-350 is deployed. You will do this to observe the configuration and familiarize yourself with the data structure. Normally, communication with the probe (including data logging) will be carried out by a data logger.

Vegetronix translator only: Use transparent mode

If using the Vegetronix translator, queries must be sent in transparent mode. This is accomplished by preceding the command with a capital T and a space. So, instead of ?!, you'll send T ?!.

Get the address

To request the address of the sensor connected to the SDI-12 bus, enter the following. Remember the T if using the Vegetronix translator.

  • Command: ?!

  • Reply:0

The number in the reply (0 in this case) is the connected sensor's address. Replies are presented in the same line as the command.

Device information

Each probe is configured with an address of 0 by default. The first time you connect to a probe, it should be the only device attached to the SDI-12 array. After connecting initially, you can set the address and then power up the remaining SDI-12 devices.

  • Command: 0I! (the address (0), the capital letter I for information, and an exclamation point). Remember the T if using the Vegetronix translator.

  • Reply: 014LI-COR LI-350034. ######### (identifying information for the sensor, including the device address, manufacturer, model, firmware version, and serial number)

Sensor reply.

Change the address

LI-350s can be addressed with alphanumeric characters (0 to 9, A to Z, and a to z). To change a sensor's address, the command starts with the current address followed by a capital A for address, and then the new address.

  • Command: 0A1! (the address (0), capital letter A for address, the number 1, which is the new address, and the exclamation point)

  • Reply: 1 (the new address)

Query the soil type setting

Mineral soil is the default soil setting. To see the current soil type setting, enter the following the command.

  • Command: 0XST! (the address (0), the command XST, and the exclamation point !)

  • Reply: Mineral_Soil (the soil type)

Change the soil type setting

To change the soil type setting, enter the command, with the new soil type.

  • Command: 0XSTM! (the sensors address (0), the command XST, and the exclamation point !), where M represents the new soil type (mineral, in this case).

  • Reply: Mineral_Soil (the soil type)

Soil type options are M for mineral soil, SL for soil-less media, C1 for custom 1, and C2 for custom 2. Custom soil types require custom calibration coefficients to be loaded, as described in Custom calibrations for the LI-350.

Get new data

Enter the M command to instruct the LI-350 to perform a new measurement.

  • Command: 0M! (address (0), the command M, ending with !).

  • Reply: 00003

Display the new data

Enter the following to report the new data.

  • Command: 0D0! (address (0), the command D0 for data group 0, ending with !.

  • Reply: The data (#, ####, ##.##, #) Sensor Address, Calibrated Counts (Moisture; signed 32-bit integer), Soil Temperature (°C), Electrical Conductivity (µS/cm).

To read the latest data, do both Get new data and Display the new data.

Basic SDI-12 commands

With the letter a as a proxy for sensor address, you can send the following commands to the LI-350. Remember the T if using the Vegetronix translator. Refer to the documentation for your adapter for more details.

Sensor information commands

Table 4‑2. Basic commands can query the sensor and set some basic settings, such as soil type.
Command Description Expected Response
aI! Identify sensor LI-COR LI-350 + SW_Version.Serial_Number
aXST! Query soil type Returns current soil type - String
aXST<char>! Set soil type
M - Mineral_Soil
SL - Soilless_Media
C1 - Custom1
C2 - Custom2		 Returns soil type stored in memory after update - String

Report data commands

Table 4‑3. Summary of commands that report data.
Command Description Expected Response
aM! Retrieve new data a0003!
aD0! Report the data Address <ASCII>
Soil Moisture - Calibrated Counts <int32>
Temperature - °C <float>
EC - µS/cm (temperature corrected) <int32>
aM1! Retrieve new data (group 1) a0005!
aD0! Report the data Address <ASCII>
VWC - m3/m3 <float>
EC - µS/cm (temperature corrected) <int32>
Temperature - °C <float>
Temperature - °F <float>
EC - µS/cm (no temperature correction) <int32>
aM2! Retrieve new data (group 2) a0009!
aD0! Report the data Address <ASCII>
VWC - m3/m3 <float>
Temperature - °C <float>
EC - µS/cm (temperature corrected) <int32>
Raw Counts <int32>
aD1! Report the data Address <ASCII>
Calibrated Counts <int32>
Soil Moisture Zero <int32>
Soil Moisture Span <float>
EC - µS/cm (no temperature correction) <int32>
EC Span <float>
aM3! Retrieve new data (group 3) a0006!
aD0! Report the data Address <ASCII>
VWC - m3/m3 - Mineral Soil <float>
VWC - m3/m3 - Soil-less Media <float>
Temperature - °C <float>
EC - µS/cm (temperature corrected) <int32>
Dielectric Permittivity - unitless <float>
Calibrated Counts <int32>

Extended SDI-12 Commands

Additional commands allow you to adjust the calibration of the sensor. These should be used with caution, as some of these command will adjust the sensor response.

Endmember calibration commands

Table 4‑4. Commands related to air and water calibrations (endmembers).
Command Description Expected Response
aXMA! Perform an air calibration ACK (NOTE: this changes the sensor response!)
aXMW! Perform a water calibration ACK (NOTE: this changes the sensor response!)
aXSA! Query air calibration value Current air calibration value - Int
aXSA<(int)>! Set air calibration value Air calibration stored in memory after update - Int
aXSW! Query water calibration value Current water calibration value - Int
aXSW<(int)>! Set water calibration value Water calibration stored in memory after update - Int

Electrical conductivity correction commands

Table 4‑5. Commands related to electrical conductivity correction.
Command Description Expected Response
aXCA! Query A electrical conductivity correction Current A factor for correction
SMS += A × (EC)2
aXCA<(float)>! Set A electrical conductivity correction Value stored in memory after update - Float
aXCB! Query B electrical conductivity correction Current B factor for correction
SMS += B × (EC)
aXCB<(float)>! Set B electrical conductivity correction Value stored in memory after update - Float

Electrical conductivity commands

Table 4‑6. Commands related to electrical conductivity calibration coefficients.
Command Description Expected Response
aXEA! Query coefficient of electrical conductivity EC3 factor Current electrical conductivity A factor - Float
aXEA<(float)>! Set coefficient of electrical conductivity EC3 squared factor Electrical conductivity A factor stored in memory after update - Float
aXEB! Query coefficient of electrical conductivity EC2 factor Current electrical conductivity B factor - Float
aXEB<(float)>! Set coefficient of electrical conductivity EC2 factor Electrical conductivity B stored in memory after update - Float
aXEC! Query coefficient of electrical conductivity EC1 factor Current electrical conductivity C factor - Float
aXEC<(float)>! Set coefficient of electrical conductivity EC1 factor Electrical conductivity C stored in memory after update - Float
aXES! Query electrical conductivity span Current value of electrical conductivity span - Scientific Notation
aXES<float>! Set electrical conductivity span Electrical conductivity span stored in memory after update - Scientific Notation
aXEK! Query electrical conductivity span Current value of electrical conductivity span - Scientific Notation
aXEK<float>! Known electrical conductivity, Calculate span;
Calculates the EC span using a user-provided EC value at 25 °C. Value is normalized using measured temperature		 Electrical conductivity span stored in memory after update - Scientific Notation

Volumetric water content commands

Table 4‑7. Commands related to volumetric water content calculations. VWC coefficients can only be changed when using a custom soil type.
Command Description Expected Response
aXVA! Query coefficient of soil a4 factor Current volumetric water content a4 factor - Float
aXVA<(float)>! Set coefficient of soil a4 factor Volumetric water content a4 stored in memory after update - Float
aXVB! Query coefficient of soil a3 factor Current volumetric water content a3 factor - Float
aXVB<(float)>! Set coefficient of soil a3 factor Volumetric water content a3 stored in memory after update - float
aXVC! Query coefficient of soil a2 factor Current volumetric water content a2 factor - Float
aXVC<(float)>! Set coefficient of soil a2 factor Volumetric water content a2 stored in memory after update - Float
aXVD! Query coefficient of soil a1 factor Current volumetric water content a1 factor - Float
aXVD<(float)>! Set coefficient of soil a1 factor Volumetric water content a1 stored in memory after update - Float
aXVE! Query coefficient of soil offset Current volumetric water content offset - Float
aXVE<(float)>! Set coefficient of soil offset Volumetric water content offset stored in memory after update - Float

Dielectric permittivity commands

Table 4‑8. Commands related to dielectric permittivity calculations.
Command Description Expected Response
aXDA! Query coefficient of dielectric permittivity E3 factor Current value of coefficient - Scientific Notation
aXDA<float>! Set coefficient of dielectric permittivity E3 factor Set coefficient value stored in memory - Scientific Notation
aXDB! Query coefficient of dielectric permittivity E2 factor Current value of coefficient - Scientific Notation
aXDB<float>! Set coefficient of dielectric permittivity E2 factor Set coefficient value stored in memory - Scientific Notation
aXDC! Query coefficient of dielectric permittivity E1 factor Current value of coefficient - Scientific Notation
aXDC<float>! Set coefficient of dielectric permittivity E1 factor Set coefficient value stored in memory - Scientific Notation
aXDD! Query coefficient of dielectric permittivity E0 factor Current value of coefficient - Scientific Notation
aXDD<float>! Set coefficient of dielectric permittivity E0 factor Set coefficient value stored in memory - Scientific Notation

Note: Coefficients have a resolution of eight digits. V commands can only be edited if the device is configured for Custom 1 (C1) or Custom 2 (C1) soil types.

Responses may include the following error codes.

Code Reply Description
00 - No error
01 UNKNOWN_CMD Unknown command
02 INVALID_INPUT Invalid input
03 EEPROM_WRITE_FAIL Write failure
04 INPUT_OUTSIDE_BOUNDS Value is outside allowed bounds
  CANNOT_OVERRIDE_PRESETS Some presets cannot be changed
99 UNKNOWN_ERROR Unknown error

The SDI-12 implementation follows the specifications at https://www.sdi-12.org/current_specification/SDI-12_version-1_4-Jan-30-2021.pdf.