Weather station with ModBus over RS-485

Abstract

A weather station build around a SparkFun Weather Meter Kit (SEN-15901). The temperature, humidity and pressure are measured with I2C sensors housed in an RS1 Passive Radiation Shield from Garni. The data can be retrieved via an RS485 ModBus RTU interface. The main processor is an Arduino Pro Mini (ATmega328P 5V@16MHz)

SparkFun SEN-15901 Weather Station

Garni RS1 Passive Radiation Shield

Sensors

• SparkFun SEN-15901 Weather Station (wind speed, wind direction and rain fall)
• Silicon Labs Si7021 (humidity and temperature)
• Bosch BMP280 (pressure and temperature)

Measurements

• Wind direction
• Wind speed (average of last 10 minutes)
• Wind gust (last 10 minutes)
• Rain fall (last hour)
• Rain fall (last 24 hours)
• Temperature
• Humidity
• Atmospheric pressure

ModBus

• RS485 transceiver
• ModBus address: 14 (fixed in software)

Input registers

The measurments and order of the measurements are the same as for APRS weather reports. But ofcourse we use SI units.

Address	Description	Units
30001	Wind direction	degrees
30002	Wind speed (average of 10 minutes)	m/s * 100
30003	Wind gust (peak of last 10 minutes)	m/s * 100
30004	Temperature	degrees Celcius * 100
30005	Rain last hour	l/m2 * 100
30006	Rain last 24 hours	l/m2 * 100
30007	Rain since midnight	NOT IMPLEMENTED
30008	Humidity	percent * 100
30009	Barometric pressure	hPa * 100

The ModBus registers are 16 bit wide. For better precision, some units are scaled by a factor of 100. This way, values with up to two decimal points can be stored as 16 bit integer values. Just divide by 100 to get the floating point values.

Dependencies

• Arduino IDE

Arduino libraries

• https://github.com/sparkfun/SparkFun_Weather_Meter_Kit_Arduino_Library
• https://github.com/orgua/iLib
• https://github.com/epsilonrt/modbus-arduino
• https://github.com/epsilonrt/modbus-serial

Libraries are included with the source code of this project

License

Copyright (C) 2023 M.T. Konstapel

Software

This program is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version.

Hardware and documentation

This work is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.