Apogee Instruments PYRANOMETER SP-421 Owner's Manual - Page 12
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Sensor Calibration
The SP-421 pyranometer has 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 SP-421
pyranometers. 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
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
SP-421 pyranometers 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 SP-421 Pyranometers
Instruction Name
Send Identification Command
Measurement Command
Measurement Command w/ Check
Character
Change Address Command
Concurrent Measurement Command
Concurrent Measurement Command w/
Check Character
Address Query Command
Get Data Command
Make Measurement Command: M!
The make measurement command signals a measurement sequence to be performed. Data values generated in response to
this command are stored in the sensor's buffer for subsequent collection using "D" commands. Data will be retained in sensor
storage until another "M", "C", or "V" command is
executed. M commands are shown in the following examples:
Command
Response
aM! or aM0!
a0011<cr><lf>
aM1!
a0011<cr><lf>
where a is the sensor address ("0-9", "A-Z", "a-z") and M is an upper-case ASCII character.
The data values are separated by the sign "+", as in the following example (0 is the address):
(version 1.4, August 10, 2016).
Instruction Syntax
aI!
Send identification information
aM!
Tells the sensor to take a measurement
Tells the sensor to take a measurement and return it
aMC!
with a check character
aAb!
Changes the address of the sensor from a to b
Used to take a measurement when more than one
aC!
sensor is used on the same data line
Used to take a measurement when more than one
aCC!
sensor is used on the same data line. Data is returned
with a check character.
Used when the address is unknown to have the sensor
?!
identify its address
aD0!
Retrieves the data from a sensor
Response to 0D0!
2
Returns watts/m
Returns millivolt output
Description
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