User Manual

Sensor Calibration
All Apogee SDI-12 SQ-400 series quantum sensors have sensor-specific calibration coefficients determined during
the custom calibration process. Coefficients are programmed into the microcontrollers at the factory.
SDI-12 Interface
The following is a brief explanation of the serial digital interface SDI-12 protocol instructions used in Apogee SQ-
421 quantum sensors. For questions on the implementation of this protocol, please refer to the official version of
the SDI-12 protocol: http://www.sdi-12.org/specification.php (version 1.4, August 10, 2016).
Overview
During normal communication, the data recorder sends a packet of data to the sensor that consists of an address
and a command. Then, the sensor sends a response. In the following descriptions, SDI-12 commands and
responses are enclosed in quotes. The SDI-12 address and the command/response terminators are defined as
follows:
Sensors come from the factory with the address of “0” for use in single sensor systems. Addresses “1 to 9” and
“A to Z”, or “a to z”, can be used for additional sensors connected to the same SDI-12 bus.
“!” is the last character of a command instruction. In order to be compliant with SDI-12 protocol, all commands
must be terminated with a “!”. SDI-12 language supports a variety of commands. Supported commands for the
Apogee Instruments SQ-421 quantum sensors are listed in the following table (“a” is the sensor address. The
following ASCII Characters are valid addresses: “0-9” or “A-Z”).
Supported Commands for Apogee Instruments SQ-421 Quantum Sensors
Instruction Name
Instruction Syntax
Description
Acknowledge Active Command
a!
Responds if the sensor with address a is on the line
Send Identification Command
aI!
Responds with sensor information
Measurement Command
aM!
Tells the sensor to take a measurement
Measurement Command w/ Check
Character
aMC!
Tells the sensor to take a measurement and return it
with a check character
Change Address Command
aAb!
Changes the sensor address from a to b
Concurrent Measurement Command
aC!
Used to take a measurement when more than one
sensor is used on the same data line
Concurrent Measurement Command w/
Check Character
aCC!
Used to take a measurement when more than one
sensor is used on the same data line. Data is returned
with a check character.
Address Query Command
?!
Used when the address is unknown to have the sensor
identify its address, all sensors on data line respond
Get Data Command
aD0!
Retrieves the data from a sensor