tMTH-4 - Temperature and humidity meter with Modbus interface

tMTH-4 is a temperature and humidity measurement module with Modbus RTU interface. It features four independent I²C buses, dedicated for connecting AHT20/AM2301B temperature and humidity sensors, allowing for a total of 4 sensors.
Temperature and humidity data is made available via Modbus registers.

The tMTH-4 is primarily designed to function as an extension module for LK4 or LK3.5+ devices (also compatible with LK3.9), enabling scalable and distributed temperature and humidity monitoring.
A ready-to-use configuration file for these devices is available, allowing for quick and hassle-free integration.

Basic features¶

Supports up to 4 AHT20/AM2301B temperature and humidity sensors (via 4 independent I²C buses).
Provides sensor readings via the Modbus RTU protocol.
Compatible with LK4, LK3.5+, and LK3.9 devices.
Can be connected to a PC via USB to access the configuration console (described in a later section).

Technical specifications¶

Parameter	Value
Power supply	5-12 V DC (connector) 5 V (USB)
Sensor support	Up to 4 × AHT20/AM2301B sensors
Interfaces	I²C: 4 buses via RJ12 connectors, Modbus RTU (RS485), Serial (over USB; 115200 baud)
Dimensions	74 × 40 × 30 mm (without DIN handle)
Weight	50 g

Default setup¶

Out of the box, the tMTH-4 is pre-configured with:

Modbus Slave ID: 1
I²C bus frequency: 20 kHz for all 4 inputs

These defaults suit most standard installations. However, if you need to change the Modbus address see section below or if you need to change other settings, refer to the Advanced Configuration section.

Quick Modbus address configuration¶

If you need to change the Modbus address, you can do so using the built-in function button and the LED located on the PCB. The button is placed near the edge of the board opposite the side with the USB port and the Modbus and power supply connectors.

To read the current address:

Click the button once.
The LED will blink to indicate the address.
(For example, 2 blinks = address 2.)

To set a new address:

Press and hold the button.
After 2 seconds, the LED will blink once.
Continue holding — the LED will blink once per second up to 5 times.
Release the button after the number of blinks that corresponds to your desired address.
(For example, for address 3, release after the third blink.)

⚠️ If you hold the button for more than 7 seconds, the process is canceled and no changes are made.

For additional configuration options, see the Advanced Configuration section.

Integration with LK¶

The tMTH-4 integrates seamlessly with LK4 and LK3.5+ devices. It can be powered directly from the LK controller using its 5 V and GND outputs. Then, connect it—either alone or together with other Modbus devices—to the LK’s Modbus A+ and B− terminals to establish communication.

Once connected, configure a custom Modbus module on the LK using the ready-to-use configuration preset available in the Downloads section. You can easily modify this preset to read only the parameters you need, such as selected temperature and humidity sensors.

Firmware update¶

To update the firmware, the device must be placed into bootloader mode. In this mode, the device appears as a mass storage device named RPI-RP2 when connected to a PC. Simply copy the new firmware file to that storage. After the upload, the device will automatically restart and resume normal operation with the new firmware.

Enabling bootloader mode¶

You can enable bootloader mode in one of two ways:

Via USB Console
Connect the device to a PC and open the serial console (see instructions in Advanced Configuration).
Then, send the command bootloader.
The connection will be interrupted, and the device will restart in bootloader mode.
Via JP2 jumper pad (hardware method)
Open the device casing to access the JP2 jumper pad — two adjacent contact fields on the PCB.
Temporarily short the two pads (for example using tweezers or another metal tool) before connecting the device to the PC via USB.
Keep the pads shorted briefly as the device powers on. Once it enters bootloader mode, remove the short.

On older hardware revisions, a FLASH / SW button located on the PCB is used instead.
Press and hold the button before connecting the device to the PC via USB, then release it once the device enters bootloader mode.

Firmware files are available in the Downloads section.

Advanced configuration¶

To modify the settings, you will need to connect the tMTH-4 to a computer via USB. The device can be powered through USB, so no additional power connections are necessary.

Required software¶

To establish a connection with the board, you’ll need an application that can handle serial communication. Popular choices include:

PuTTy
TeraTerm
PySerial (Python library), which includes useful tools like pyserial-miniterm (for managing connections) and pyserial-ports (for listing available COM ports).

Example Configuration Process¶

List Available Ports
Run pyserial-ports to list the available COM ports before connecting the device. The output might look like:
```
COM1
COM3
COM15
COM16
COM17
```
Connect the Device
Plug in the tMTH-4 and run the command again. The new COM port (e.g., COM11) will represent the connected device.
Connect to tMTH-4
Use the following connection parameters:
- Baud rate: 115200
- Byte size: 8 bits
- Parity: None
- Stop bits: 1
Example command:
```
pyserial-miniterm COM11 115200
```
Send Commands
Once connected, you can issue commands to the device. Each command must be followed by the Enter key. A useful command is ? or help, which provides a list of all available commands (described below).

Command list¶

Command	Description
address? \| a?	Get the Modbus address (Slave ID).
address=X \| a=X	Set the Modbus address. Parameters: X - address to set, number `<1, 255>`.
frequency? \| f?	Get the I²C bus frequency (for all interfaces), value in Hz.
frequency=X \| f=X	Set the I²C bus frequency in Hz. Parameters: X - frequency to set, number `<10_000, 400_000>`.
read_sensors \| rs	Read the sensors readings.
init_sensors \| is	Detect and initialize the sensors.
save_config \| sc	Save configuration to Flash memory. It should be called after changing the Modbus address.
read_registers \| rr	Read Modbus register values.
read_definitions \| rd	Read Modbus register definitions (name, address, type).
bootloader	Reboot the device into bootloader mode to update the firmware. When connected to a computer, it will appear as a storage device named `RPI-RP2`, to which you should copy the firmware file with the extension `uf2`. Then the device will reboot into normal operation mode.
restart	Restart the device.
verbose=X \| v=X	Turn on or off verbose mode, which displays more messages.
help \| ?	Display this help message with a list of available commands.

Common use cases¶

Some commands are useful only in specific situations.

Modbus addressing
When using multiple tMTH-4 devices on one Modbus bus or together with another device that has the same address, it is recommended to change the address using the command address=X so that each device has a unique one.

Detecting connected sensors
Sensors are detected and initialized when the device is started. To initialize sensors connected later, use the init_sensors command.

Saving changes
After changing settings using the commands address=X or frequency=X, you must use save_config to store the configuration in permanent memory. Otherwise, the changes will be lost after a power loss or device restart.

Modbus registers¶

Below is the list of Modbus registers available in the device.
The content corresponds to the output of the read_definitions command and includes register names, addresses, types, and divisors.

| Name                           | Addr | Type   | Func         | Divisor |
| ------------------------------ | ---- | ------ | ------------ | ------- |
| uptime                         | 3000 | uint32 | 0x04 (IREG)  |         |
| temperature1                   | 3002 | int32  | 0x04 (IREG)  |      10 |
| humidity1                      | 3004 | int32  | 0x04 (IREG)  |      10 |
| temperature2                   | 3006 | int32  | 0x04 (IREG)  |      10 |
| humidity2                      | 3008 | int32  | 0x04 (IREG)  |      10 |
| temperature3                   | 3010 | int32  | 0x04 (IREG)  |      10 |
| humidity3                      | 3012 | int32  | 0x04 (IREG)  |      10 |
| temperature4                   | 3014 | int32  | 0x04 (IREG)  |      10 |
| humidity4                      | 3016 | int32  | 0x04 (IREG)  |      10 |
| version_major                  | 3100 | uint16 | 0x04 (IREG)  |         |
| version_minor                  | 3101 | uint16 | 0x04 (IREG)  |         |
| version_patch                  | 3102 | uint16 | 0x04 (IREG)  |         |
| address                        | 4000 | uint16 | 0x03 (HREG)  |         |
| frequency                      | 4001 | uint32 | 0x03 (HREG)  |         |