OPERATION AND MEASUREMENT
The SQ-627 Extended Range PFD sensor has a SDI-12 output, where photosynthetically active radiation is returned
in digital format. Measurement of SQ-627 Extended Range PFD sensors requires a measurement device with SDI-
12 functionality that includes the M or C command.
Wiring
Sensor Calibration
All Apogee SDI-12 Extended Range PFD sensor models (SQ-600 series) 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-
627 Extended Range PFD 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 wi
th 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 SD
I-12 protocol, all commands
must be terminated with a “!”. SDI
-12 language supports a variety of commands. Supported commands for the
Apogee Instruments SQ-627 Extended Range PFD
sensors are listed in the following table (“a” is the sensor
address. The fo
llowing ASCII Characters are valid addresses: “0
-
9” or “A
-
Z”).
White: Positive (signal from sensor)
Red: Input Power
Black: Ground (from sensor signal and output power)
Clear: Shield/Ground