Arduino bridge to present I2C sensor values on a Modbus network.
Tested with Arduino Pro Mini (328p) but other boards should have success.
- SHT31 - Humidity and temperature sensor.
- SGP30 - Multi-gas (VOC and CO₂eq) sensor.
- Uses software serial and I2C libraries so any hardware pins can be used (search '_PIN' in code to change).
- Instantaneous and moving average (configurable period) for all values.
- Delta (relative change over period) values for humidity/temperature (configurable period).
- Automatic detection of connected sensors.
- Automatic disabling of a failed sensor (read of input registers for failed/missing sensor return Modbus exception). Reboot/power cycle microcontroller after replacing failed sensors.
- SGP30 baseline is periodically saved in EEPROM and used to initialise same device (serial number check) on power up.
- SGP30 humidity compensation occurs when SHT31 sensor is also available.
Updating any of these writeable registers causes an instant 'restart' (re-initialisation).
| # | Use | R/W | Native Range | Datatype | Default |
|---|---|---|---|---|---|
| 0 | Software Version | R | 16 bit unsigned int | Version | |
| 1 | Device address | R/W | 1 - 247 | 16 bit unsigned int | 1 |
| 2 | Baud rate | R/W | 2400 - 57600 | 16 bit unsigned int | 9600 |
| 3 | SHT31 polling Frequency (ms) | R/W | 0 - 65535 | 16 bit unsigned int | 1000 |
| 4 | SHT31 moving average period (s) | R/W | 0 - 65535 | 16 bit unsigned int | 30 |
| 5 | SHT31 delta period (s) | R/W | 0 - 65535 | 16 bit unsigned int | 30 |
| 6 | SGP30 polling Frequency (ms) | R/W | 0 - 65535 | 16 bit unsigned int | 1000 |
| 7 | SGP30 moving average period (s) | R/W | 0 - 65535 | 16 bit unsigned int | 120 |
| 8 | SGP30 Serial #1 | R | 16 bit unsigned int | From SGP30 | |
| 9 | SGP30 Serial #2 | R | 16 bit unsigned int | From SGP30 | |
| 10 | SGP30 Serial #3 | R | 16 bit unsigned int | From SGP30 | |
| 11 | SGP30 eCO2 Baseline | R | 16 bit unsigned int | From SGP30 | |
| 12 | SGP30 TVOC Baseline | R | 16 bit unsigned int | From SGP30 |
| # | Device | Reading | Native Range | Datatype | Scale |
|---|---|---|---|---|---|
| 0 | SHT31 | Temperature | -40 - 125°C | 16 bit signed int | x 100 |
| 1 | SHT31 | Humidity | 0 - 100% | 16 bit unsigned int | x 100 |
| 2 | SHT31 | Temperature (moving average) | -40 - 125°C | 16 bit signed int | x 100 |
| 3 | SHT31 | Humidity (moving average) | 0 - 100% | 16 bit unsigned int | x 100 |
| 4 | SHT31 | Temperature delta | -165 - 165°C | 16 bit signed int | x 100 |
| 5 | SHT31 | Humidity delta | -100 - 100% | 16 bit signed int | x 100 |
| 6 | SGP30 | Raw H2 | 16 bit unsigned int | ||
| 7 | SGP30 | Raw ethanol | 16 bit unsigned int | ||
| 8 | SGP30 | eCO2 | 0 - 1000ppm | 16 bit unsigned int | |
| 9 | SGP30 | TVOC | 0 - 1000ppb | 16 bit unsigned int | |
| 10 | SGP30 | IAQ index | 16 bit unsigned int | ||
| 11 | SGP30 | eCO2 (moving average) | 0 - 1000ppm | 16 bit unsigned int | |
| 12 | SGP30 | TVOC (moving average) | 0 - 1000ppb | 16 bit unsigned int | |
| 13 | SGP30 | IAQ index (moving average) | 16 bit unsigned int |
Default pin configurations is:
- Modbus RX -> pin 16 (A2)
- Modbus TX -> pin 17 (A3)
- I2C Clock -> pin 15 (A1)
- I2C Data -> pin 14 (A0)
Note: Examples assume Linux modpoll with USB RS485 on /dev/ttyUSB0.
Default Modbus address is 1. Even without sensors connected the device should respond to read all holding registers:
modpoll -m rtu -b 9600 -p none -a 1 -0 -1 -t 4 -r 0 -c 13 /dev/ttyUSB0
To change Modbus address from 1 to 2:
modpoll -m rtu -b 9600 -p none -a 1 -0 -1 -t 4 -r 1 -c 1 /dev/ttyUSB0 2
To change SHT31 delta period to 60 seconds:
modpoll -m rtu -b 9600 -p none -a 1 -0 -1 -t 4 -r 5 -c 1 /dev/ttyUSB0 60
To read all input registers (must have all sensors connected or a Modbus exception will be returned):
modpoll -m rtu -b 9600 -p none -a 1 -0 -1 -t 3 -r 0 -c 14 /dev/ttyUSB0
To read SHT31 input registers:
modpoll -m rtu -b 9600 -p none -a 1 -0 -1 -t 3 -r 0 -c 6 /dev/ttyUSB0
To install in a very small space, screw terminals are useful. A ProMini328P and RS485 <-> TTL module can be connected thus:
The modules can be folded back to back and held with hot glue. In this configuration re-programming can still be accomplished by using test clips on the exposed serial connections.
This yields a very small installation package. It can easily fit inside a standard European wall box:
In this example installation power & data are provided via Cat7 cable joined inside the same box using Wago 243 connectors. A wall mounted enclosure of the installer's choice finishes the job.
Some of this shamelessly copied from Adafruit libraries: