Geminox THRi

Modbus RTU

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Last updated: 03. 2026

Amit DM-OT2 Product Page

Requires external gateway
Amit DM-OT2

The Geminox THRi is a gas condensing boiler connected to TapHome through the Amit DM-OT2 OpenTherm-to-Modbus RTU gateway. The DM-OT2 translates the boiler’s OpenTherm protocol into Modbus RTU holding registers, giving TapHome access to temperature sensors, heating and DHW thermostats, burner status, and heat source power monitoring.

The template uses bitmask registers for status and control (H:100 and H:200), Float registers for temperatures and pressure, and a cyclic extra OT register reading mechanism to retrieve min/max values from OpenTherm data-IDs that are not mapped to standard registers.

Hardware connection

Gateway overview

The Amit DM-OT2 is a DIN-rail mounted converter with a galvanically isolated RS-485 interface and an OpenTherm/+ interface. It acts as an OpenTherm master, communicating with the boiler, and as a Modbus RTU slave on the RS-485 bus.

Technical specifications

Power supply: 20–30 V DC, max 30 mA
Operating temperature: -40°C to +70°C
Protocol: Modbus RTU Slave, RS-485
Baud rate: 9600, no parity, 2 stop bits
Max converters: 63 on a single RS-485 network
RS-485 isolation: Galvanic, 500 V AC
Mounting: 35 mm DIN rail

Terminal connections

Terminal	Signal	Description
1	G485	RS-485 ground
2	B	RS-485 signal B (-)
3	A	RS-485 signal A (+)
4	GND	Power supply ground
5	+24V	Power supply +24 V DC
6	RC1	OpenTherm/+ interface
7	RC2	OpenTherm/+ interface

RS-485 wiring to TapHome

Connect using a 3-wire RS-485 connection:

A (terminal 3) → A+/D+ on TapHome RS-485
B (terminal 2) → B-/D- on TapHome RS-485
G485 (terminal 1) → GND on TapHome power supply (mandatory)

Important — Grounding
The G485 ground wire must be connected between the DM-OT2 and the TapHome power supply. Without proper grounding, communication errors may occur.

Bus termination

The DM-OT2 has jumpers for RS-485 termination:

Jumpers fitted — terminal station (idle states and terminations active). Use on the last device on the bus.
Jumpers not fitted — intermediate station. Use on all other devices.

Both jumpers must always be set simultaneously.

Configuration

DIP switch addressing

DIP switches ADR0–ADR5 (switches 1–6) set the Modbus slave address (range 1–63). Address 0 is not permitted.

Address calculation: Each switch represents a binary value — ADR0=1, ADR1=2, ADR2=4, ADR3=8, ADR4=16, ADR5=32. Sum the values of switches in the ON position.

Address examples:

Address	Switches ON
1	ADR0
2	ADR1
3	ADR0 + ADR1
5	ADR0 + ADR2
10	ADR1 + ADR3

Recommended: Address 1 (ADR0=ON, all others OFF).

Communication speed

DIP switches COMM0–COMM3 (switches 7–10) configure the baud rate and parity. For TapHome, set:

COMM0: OFF, COMM1: OFF, COMM2: ON — this selects 9600 baud, no parity

With no parity selected, the DM-OT2 automatically uses 2 stop bits. Changes take effect immediately.

Protocol selection

DIP switch MODE (switch 11) selects the protocol:

Not fitted (OFF) — Modbus RTU (required for TapHome)
Fitted (ON) — ARION protocol

A change takes effect after a restart.

Device capabilities

Status monitoring

Three binary status sensors read individual bits from the Status register (H:100):

Burner (bit 4) — indicates whether the burner flame is active
Heating (bit 2) — indicates whether central heating is active
DHW Heating (bit 3) — indicates whether domestic hot water heating is active

These sensors update every 2500 ms and provide real-time visibility into the boiler’s operating state.

Heating and DHW control

Two enable switches control heating and DHW via the Control register (H:200):

Heating Enable (bit 5) — enables or disables central heating
DHW Enable (bit 6) — enables or disables domestic hot water heating

Both switches use a read-modify-write pattern: the template reads the full H:200 register, masks out the target bit, sets the new value, and writes back. A counter-based debounce (cnt=5 polling cycles) ensures the switch shows the written state immediately after toggling, preventing flickering while the boiler processes the command.

Temperature thermostats

Two thermostat devices provide both current temperature reading and setpoint control:

Heating Circuit Temperature — reads the current flow temperature from H:102 (Float) and reads/writes the desired temperature to H:201 (Float). A service attribute shows the min/max setpoint range from OpenTherm data-ID 49.
DHW Thermostat — reads the current DHW temperature from H:106 (Float) and reads/writes the desired temperature to H:205 (Float). A service attribute shows the min/max setpoint range from OpenTherm data-ID 48.

Temperature sensors

Three read-only temperature sensors provide additional monitoring:

Flue Gas Temperature (H:118) — exhaust gas temperature from the boiler, useful for monitoring combustion efficiency
Return Water Temperature (H:108) — return water temperature from the heating circuit
Outdoor Temperature (H:110) — outdoor ambient temperature from the boiler’s sensor, used for weather-compensated heating curves

All temperature registers use 32-bit Float format (2 consecutive holding registers).

Pressure monitoring

Heating Circuit Pressure (H:114) — water pressure in the heating circuit in bar. Low pressure may indicate a water leak or the need to refill the system.

Heat source power

Heat Source Power — a dimmer model representing the boiler’s modulation level. Two service attributes from OpenTherm data-ID 15 show the minimum modulation percentage and the maximum boiler power in kW.

Extra OpenTherm register reading

The template includes a cyclic mechanism for reading OpenTherm data-IDs that are not directly mapped to standard Modbus registers. This is used to retrieve min/max temperature ranges and boiler capacity data.

How it works:

The module ReadScript enables extra OT register reading by setting bit 10 of H:200
It writes the desired OpenTherm data-ID to H:216 and triggers a read via H:217
The response appears in H:120, with status in H:121 (1=success, 3=error)
The script cycles through three data-IDs: 48 (DHW min/max), 49 (heating min/max), and 15 (boiler capacity)

The retrieved values are stored in internal variables (TUV_MM, Top_MM, Kotol_MM) and made available as service attributes on the corresponding devices.

Service diagnostics

The module exposes nine service attributes for monitoring:

Gateway ID (H:0) — confirms the gateway type (197 = DM-OT2)
Firmware Version (H:1) — current gateway firmware version
Time (H:2) — gateway uptime counter
Guard Time (H:4) — Modbus communication failure evaluation timeout in milliseconds
Reset Counter (H:8) — number of gateway resets (resettable via service action)
Modbus Message Counter (H:9) — total Modbus messages processed (resettable via service action)
DHW Present (H:100, bit 6) — whether DHW heating capability is available
TV2 Present (H:100, bit 7) — whether a secondary heating circuit is available
Heat Source Power [%] (H:112) — current boiler modulation as a percentage

Two service actions allow resetting the gateway counters:

Reset Reset Counter — clears the gateway reset counter (H:8)
Reset Modbus Message Counter — clears the Modbus message counter (H:9)

The module ReadScript also monitors OpenTherm communication status (H:100 bit 0) and device errors (H:100 bit 1, H:101), reporting them as errors and warnings in TapHome.

LED indicators

LED	Function
PWR	Converter is powered
RxD	Receiving data on RS-485
TxD	Transmitting data on RS-485
RxOT	Receiving data on OpenTherm
TxOT	Transmitting data on OpenTherm

Normal state: PWR on, RxD/TxD blink during Modbus communication, RxOT/TxOT blink during OpenTherm communication.

Troubleshooting

No Modbus communication (RxD/TxD LEDs not blinking)

Check RS-485 cables:
- A (terminal 3) correctly connected to A+/D+ on TapHome
- B (terminal 2) correctly connected to B-/D- on TapHome
- G485 (terminal 1) connected to TapHome power supply ground — most common issue
Verify the Modbus slave address on DIP switches ADR0–ADR5
Confirm DIP switch MODE is OFF (Modbus RTU selected)
Check the communication speed settings (COMM0=OFF, COMM1=OFF, COMM2=ON for 9600 baud)
Confirm TapHome is set to: 9600 baud, no parity, 2 stop bits

No OpenTherm communication (RxOT/TxOT LEDs not blinking)

Check the wiring between RC1/RC2 terminals and the boiler’s OpenTherm connector
Verify the boiler supports OpenTherm (not all Geminox models have an OT interface)
Check that the DM-OT2 has correct power supply (20–30 V DC)

OT communication error in TapHome

If TapHome reports “OT communication error” (Status bit 0 = 0):

Check the OpenTherm wiring between the DM-OT2 and the boiler
Restart the DM-OT2 by cycling power
Check the Guard Time service attribute — if set to 0, the connection is permanently marked as disconnected

Device error in TapHome

If TapHome reports “Device error” (Status bit 1 = 1):

Read the Last Error register value from TapHome warnings
Consult the boiler’s service manual for the specific error code
The error originates from the boiler itself, not the gateway

Additional capabilities

The DM-OT2 gateway also exposes registers for boiler supply water temperature (H:116), direct DHW temperature (H:104), and additional control bits on H:200 for OT communication enable and error acknowledgment. These can be added in a future template update.

Available devices

Geminox THRi (via Amit DM-OT2) Module

Service Attributes

Gateway ID	reads H:0 (UInt16); switch: 197=\'DM-OT2\', other=\'Unknown\'; identifies the Amit DM-OT2 gateway
Firmware Version	reads H:1 (UInt16); gateway firmware version number
Time	reads H:2 (UInt32); gateway uptime or system time counter
Guard Time [ms]	reads H:4 (UInt16); Modbus guard time in milliseconds
Reset Counter	reads H:8 (UInt16); number of gateway resets since last clear
Modbus Message Counter	reads H:9 (UInt16); total Modbus messages processed since last clear
DHW Present	reads bit 6 of Status (H:100); \'Yes\' if DHW heating is available, \'No\' otherwise
TV2 Present	reads bit 7 of Status (H:100); \'Yes\' if secondary heating circuit (TV2) is available, \'No\' otherwise
Heat Source Power [%]	reads H:112 (Float); current boiler modulation/power output as percentage

Service Actions

Reset Reset Counter	writes H:8 (UInt16) = 0; clears the gateway reset counter
Reset Modbus Message Counter	writes H:9 (UInt16) = 0; clears the Modbus message counter

Geminox THRi (cez Amit DM-OT2)

Read (module)

Status := modbusr(H, 100, Uint16);
if(getbit(Status, 0) = 0, adderror("Chyba komunikacie OT"));
if(getbit(Status, 1) = 1, adderror("Chyba zariadenia OT"));

var lastError := modbusr(H, 101, Uint16);
if(lastError != 0, addwarning("Chyba zariadenia " + lastError));

# povol citanie extra OT registrov
var ret := modbusr(H, 200, Uint16);
if getbit(ret, 10) = 0
    modbusw(H, 200, Uint16, ret + (1 << 10));
end

# citanie extra OT registrov
var reg := modbusr(H,216,Uint16);
var stat := modbusr(H,121,Uint16);
var out := 0;
if reg = 48
    #TUV Min/Max
    if  stat = 1
        out := modbusr(H,120,Uint16);
        TUV_MM := out;
        modbusw(H, 216, Uint16, 49);
        modbusw(H, 217, Uint16, 1);
    end
elseif reg = 49
    #kurenie Min/Max
    if stat = 1
        out := modbusr(H,120,Uint16);
        Top_MM := out;
        modbusw(H, 216, Uint16, 15);
        modbusw(H, 217, Uint16, 0);
    end
elseif reg = 15
    #kotol Min/Max
    if stat = 1
        out := modbusr(H,120,Uint16);
        Kotol_MM := out;
        modbusw(H, 216, Uint16, 48);
        modbusw(H, 217, Uint16, 1);
    end 
else
    #zaciname s TUV Min/Max
    modbusw(H, 216, Uint16, 48);
    modbusw(H, 217, Uint16, 1);
end

if modbusr(H,121,Uint16) = 3
    #ak chyba zaciname odznova s TUV Min/Max
    modbusw(H, 216, Uint16, 48);
    modbusw(H, 217, Uint16, 1);
end

Service Attributes

switch(modbusr(H, 0, Uint16),
197, "DM-OT2",
"Unknown")

Verze firmware

modbusr(H, 1, Uint16)

Čas

modbusr(H, 2, Uint32)

GuardTime [ms]

modbusr(H, 4, Uint16)

Počítadlo resetů

modbusr(H, 8, Uint16)

Počítadlo MODBUS zpráv

modbusr(H, 9, Uint16)

Ohřev TUV přítomen

if(getbit(Status, 6), "Ano", "Ne")

TV2 přítomna

if(getbit(Status, 7), "Ano", "Ne")

Výkon topného zdroje [%]

modbusr(H, 112, Float)

Service Actions

Nulovat Počítadlo resetů

modbusw(H, 8, Uint16, 0)

Nulovat Počítadlo MODBUS zpráv

modbusw(H, 9, Uint16, 0)

Burner Reed Contact Read-only

Burner flame status — active when the boiler burner is firing

Heating Reed Contact Read-only

Central heating status — active when the heating circuit is running

DHW Heating Reed Contact Read-only

Domestic hot water heating status — active when the DHW circuit is running

Heating Enable Switch

Enables or disables central heating via read-modify-write on the control register

Heating Enable

Read switch state

if cnt = 0
    var ret := getbit(modbusr(H,200,Uint16), 5);
    return (ret);
else
    cnt := cnt -1;
    return (St);
end

Write switch state

var ret := modbusr(H,200,Uint16);
var x := (ret & 0xffdf) + (St << 5);
modbusw(H, 200, Uint16, x);
cnt := 5;

DHW Enable Switch

Enables or disables domestic hot water heating via read-modify-write on the control register

DHW Enable

Read switch state

if cnt = 0
    var ret := getbit(modbusr(H,200,Uint16), 6);
    return (ret);
else
    cnt := cnt -1;
    return (St);
end

Write switch state

var ret := modbusr(H,200,Uint16);
var x := (ret & 0xffbf) + (St << 6);
modbusw(H, 200, Uint16, x);
cnt := 5;

Flue Gas Temperature Temperature Sensor Read-only

Exhaust gas temperature from the boiler combustion chamber

Return Water Temperature Temperature Sensor Read-only

Return water temperature from the heating circuit

Heating Circuit Temperature Thermostat

Heating circuit flow temperature with adjustable setpoint and min/max range from OpenTherm

Service Attributes

Heating Circuit Min / Max Temperature

Heating Circuit Temperature

Read temperature

MODBUSR(H, 102, Float)

Read desired temperature

MODBUSR(H, 201, Float)

Write desired temperature

MODBUSW(H, 201, Float, Se)

Service Attributes

Min / Max Teplota Vykurovacieho okruhu

getbyte(Top_MM, 1) + "/" + getbyte(Top_MM, 0)

Heating Circuit Pressure Variable Read-only

Water pressure in the heating circuit

DHW Thermostat Thermostat

Domestic hot water temperature with adjustable setpoint and min/max range from OpenTherm

Service Attributes

DHW Min / Max Temperature

DHW Thermostat

Read temperature

MODBUSR(H, 106, Float)

Read desired temperature

MODBUSR(H, 205, Float)

Write desired temperature

MODBUSW(H, 205, Float, Se)

Service Attributes

Min / Max Teplota TUV

getbyte(TUV_MM, 1) + "/" + getbyte(TUV_MM, 0)

Outdoor Temperature Temperature Sensor Read-only

Outdoor ambient temperature from the boiler sensor

Heat Source Power Dimmer Read-only

Boiler modulation level with min modulation percentage and max power service attributes

Service Attributes

Min Modulation [%]

Max Power [kW]

Connection: Modbus RTU • 9600 baud• 8N2 • Slave ID: $[SlaveId]

Possible improvements (5)

H:104 DHW Temperature (direct) — H:104 Float — likely DHW flow temperature; not exposed as separate device (DHW Thermostat uses H:106 for current)
H:116 Boiler Water Temperature — H:116 Float — boiler flow/supply water temperature; could complement return water temp
H:200 bit 0 OT Communication Enable — Bit 0 of H:100 indicates OT comm status; corresponding control bit on H:200 could enable/disable OT communication
H:200 bit 1 Device Error Acknowledge — Bit 1 of H:100 indicates device error; potential error acknowledgment via H:200
H:203 Heating Circuit Desired Temperature (alternative) — Potential alternative setpoint register; template uses H:201 for heating circuit desired temp

Download Template

Sources

Amit DM-OT2 Datasheet
amitomation.cz 2026-03-20
PDF
Amit DM-OT2 Product Page
amitomation.cz 2026-03-20
OpenTherm-to-Modbus Register Reference (Unipi MM-OT101)
kb.unipi.technology 2026-03-20