Gill Instruments MaxiMet GMX551
Compact weather station for wind, air temperature, humidity, air pressure, precipitation and solar radiation.
The Gill Instruments MaxiMet GMX551 is a compact weather station without moving parts that measures wind speed and direction, air temperature, humidity, air pressure, precipitation and solar radiation on a mast, for example above a greenhouse roof. This Modbus RTU template reads these values together with the compass-corrected wind direction, the dew point, the sensor status and the supply voltage into ModbusCloud.
- Protocol
- Modbus RTU
- Connection
- RS485, 19200 baud
- Registers
- 15, read only
- Brand
- Gill Instruments
- Key readings
- Air temperature · Relative humidity · Solar radiation · Wind speed and direction · Precipitation intensity and total · Air pressure · Dew point · Sensor status
- Compatible models
- MaxiMet GMX551
- Last updated
Registers (15)
15 read, 0 write
| Name | Unit | Access | Address (Hidden until you enter your email) | Function (hidden) | Type (hidden) |
|---|---|---|---|---|---|
| Air Temperature | °C | Read | |||
| Relative Humidity | %RH | Read | |||
| Solar Radiation | W/m² | Read | |||
| Wind Speed | m/s | Read | |||
| Wind Direction | ° | Read | |||
| Corrected Wind Direction | ° | Read | |||
| Precipitation Intensity | mm/h | Read | |||
| Precipitation Total | mm | Read | |||
| Barometric Pressure | hPa | Read | |||
| Dew Point | °C | Read | |||
| Sensor Status | Read | ||||
| Supply Voltage | V | Read | |||
| Device Date | Read | ||||
| Device Time | Read | ||||
| Node Letter | Read |
Loading registers…
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If a device is not in the library yet, we build the template for free as soon as we have the register data.
Use cases
- Show wind, outside temperature, humidity, air pressure, precipitation and solar radiation from the greenhouse roof together on one ModbusCloud dashboard.
- Get an email alarm when the wind speed exceeds your limit, so you can check that roof vents and screens have closed.
- Get an alarm as soon as the precipitation intensity rises above zero, for example to check the roof vents.
- Compare solar radiation and outside temperature over 365 days of history to look back on the growing season.
- Set an alarm on Sensor Status not equal to 0 to hear about a sensor fault right away.
- Watch the supply voltage to spot a weak power supply or a voltage drop on a long cable.
Installation
Mount the MaxiMet on a 30 to 58 mm pole above the greenhouse roof, free of shadow, with the north marker facing north and the pyranometer level.
Connect the unit to a PC over RS232 and open MetSet. Set Protocol to MODBUS, Interface to RS485 2-wire and Message Mode to CONT, choose RTU, address and baud rate, write the settings and power-cycle the unit.
Wire pins 4 and 6 (+) and pins 5 and 7 (-) to RS485 A and B of the ModbusCloud Gateway, pin 1 to signal ground, leave pin 8 open. Supply 5 to 30 V DC on pin 2 (+) and pin 3 (-), 10 to 30 V with heating.
In ModbusCloud, add a Modbus device with address 1, 19200 baud, 8 data bits, no parity and 2 stop bits, or the values you set in MetSet, and import this template.
Wait 10 seconds after power-up. Then check that air temperature and Sensor Status (0 = OK) arrive.
Things to watch for
The weather station does not answer. What should I check?
A new MaxiMet talks Gill ASCII over RS232. In MetSet set Protocol to MODBUS, Interface to RS485 2-wire and Message Mode to CONT, then power-cycle it. Use the address, baud rate (9600 or 19200) and parity set there; defaults are address 1, 19200 baud, no parity, 2 stop bits. Swap A and B if needed. In Modbus mode MetSet reaches the unit only through Safe Mode over RS232.
Why are values on the wrong registers after a change in MetSet?
The MaxiMet fills its holding registers in the order of its output string, and this template matches the factory default string of the GMX551. Adding, removing or moving a parameter in MetSet, a unit with the GPS option or a string that includes heater status shifts every value after that point. Set the report format back to DEFAULT, or adjust the addresses in ModbusCloud.
Why does a value read NaN or a very large number?
The MaxiMet fills a value it cannot compute with 0x7FFFFFFF: NaN for a measured value, 2147483647 for Sensor Status. An example is the corrected wind direction with the compass switched off. Also check the units in MetSet. The template assumes the default units m/s, °C, hPa, mm and W/m².
Why does the precipitation total stay at 0?
The GMX551 ships with a tip size of 0.000 mm, meant for a rain yes or no sensor. Set the tip size of the connected gauge in MetSet, for example 0.2 mm for the Kalyx tipping bucket. The total resets at power-up and at midnight of the device clock (UTC by default) unless auto-reset is off. The intensity is the rain of the last 60 minutes.
Can I connect several MaxiMet units to one RS485 bus?
Yes. Give each unit its own Modbus address (1 to 247) in MetSet with the same baud rate and parity, and add each one as a separate Modbus device in ModbusCloud with this template. RS485 2-wire allows up to 32 devices on one line. Terminate both ends of the bus.
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