Apogee SP-522 Owner's Manual - Page 9

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Apogee SP-522 Owner's Manual

OPERATION AND MEASUREMENT

The SP-522 pyranometer has a Modbus output, where shortwave radiation is returned in digital format.
Measurement of SP-522 pyranometers requires a measurement device with a Modbus interface that supports the
Read Holding Registers (0x03) function.
Wiring
The Green wire should be connected to Ground to enable RS-485 communication, or it should be connected to 12
V power for RS-232 communication. Text for the White and Blue wires above refers to the port that the wires
should be connected to.
Sensor Calibration
All Apogee Modbus thermopile pyranometers (model SP-522) has 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 SP-522 pyranometers. 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
Modbus protocol specification:
Further information can be found at:
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 to be changed by writing the Slave
Address register.
http://www.modbus.org/docs/Modbus_Application_Protocol_V1_1b3.pdf
http://www.modbus.org/specs.php
White: RS-232 RX / RS-485 Positive
Blue: RS-232 TX / RS-485 Negative
Green: Select (Switch between RS-232 and RS-485)
Black: Ground
Red: Power +12 V
(2006) and the general
(2012).