Sensor Calibration
All Apogee Modbus quantum sensors (model SQ-422X) have sensor-specific calibration coefficients determined
during the custom calibration process. Coefficients are programmed into the sensors at the factory.
Modbus Interface
The following is a brief explanation of the Modbus protocol instructions used in Apogee SQ-422X quantum sensors.
For questions on the implementation of this protocol, please refer to the official serial line implementation of the
Modbus protocol:
http://www.modbus.org/docs/Modbus_over_serial_line_V1_02.pdf
(2006) and the general
Modbus protocol specification:
http://www.modbus.org/docs/Modbus_Application_Protocol_V1_1b3.pdf
(2012).
Further information can be found at:
http://www.modbus.org/specs.php
Overview
The primary idea of the Modbus interface is that each sensor exists at an address and appears as a table of values.
These values are called Registers. Each value in the table has an associated index, and that index is used to identify
which value in the table is being accessed.
Sensor addresses
Each sensor is given an address from 1 to 247. Apogee sensors are shipped with a default address of 1. If using
multiple sensors on the same Modbus line, the sensor’s address will have t
o be changed by writing the Slave
Address register.
Register Index
Each register in a sensor represents a value in the sensor, such as a measurement or a configuration parameter.
Some registers can only be read, some registers can only be written, and some can be both read and written. Each
register exists at a specified index in the table for the sensor. Often this index is called an address, which is a
separate address than the sensor address, but can be easily confused with the sensor address.
However, there are two different indexing schemes used for Modbus sensors, though translating between them is
simple. One indexing scheme is called one-based numbering, where the first register is given the index of 1, and is
thereby accessed by requesting access to regis er 1. The other indexing scheme is called zero-based numbering,
where the first register is given the index 0, and is thereby accessed by requesting access to register 0. Apogee
Sensors use zero-based numbering. However, if using the sensor in a system that uses one-based numbering, such
as using a CR1000X logger, adding 1 to the zero-based address will produce the one-based address for the register.
Register Format:
According to the Modbus protocol specification, Holding Registers (the type registers Apogee sensors contain) are
defined to be 16 bits wide. However, when making scientific measurements, it is desirable to obtain a more precise
value than 16 bits allows. Thus, several Modbus implementations will use two 16-bit registers to act as one 32-bit
register. Apogee Modbus sensors use this 32-bit implementation to provide measurement values as 32-bit IEEE
754 floating point numbers.
Apogee Modbus sensors also contain a redundant, duplicate set of registers that use 16-bit signed integers to
represent values as decimal-shifted numbers. It is recommended to use the 32-bit values, if possible, as they
contain more precise values.
