Daikin DCOM-LT/MB Modbus interface for Daikin Altherma LT heat pumps Modbus register map

Updated 29 October 2026

Short answer

The DCOM-LT/MB is a small DIN-rail interface that sits on the Altherma P1P2 user-interface bus and exposes the heat pump as a Modbus RTU slave. Holding registers carry the commands and setpoints (on/off, mode, leaving water and room setpoints, domestic hot water, quiet mode, weather-curve offsets); input registers report the error state, running flags, temperatures and flow. Every register is one signed 16-bit word, with sensor temperatures in hundredths of a degree.

This page lists 32 Modbus registers for the Daikin DCOM-LT/MB Modbus interface for Daikin Altherma LT heat pumps — 13 writable — with the table, 0-based wire address, data type, word order, scale, units and access of each, re-typed from Daikin’s own free documentation (linked below) and checked against it.

Manufacturer: Daikin
Model: DCOM-LT/MB Modbus interface for Daikin Altherma LT heat pumps
Equipment type: Heat pump
Registers: 32 (13 writable)
Transport: Modbus RTU
Default unit ID: Not stated
Default serial: 9600 baud, 8N1
Documented addressing: 0-based (the number printed is the address on the wire)
Also known as: DCOM-LT-MB, 534-001, Altherma LT Modbus interface
Source document: DCOM-LT/MB Reference Manual (English) · 534-252-A3

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Integration notes: Daikin DCOM-LT/MB Modbus interface for Daikin Altherma LT heat pumps

Integration in brief

Wire P1P2 to the Altherma user interface and RS-485 to the master, set the slave address on DIP switches SW1.3-SW1.8, wait for the green status LED, then poll input registers 21-50 with function 04 and write setpoints with function 06 or 16.

Minimum registers to map

Addresses are 0-based wire addresses. Enough to monitor and run the unit; map more as the job needs.

Register	Name	Type	Why	Access
HR 4	Space heating/cooling on/off	INT16	Space heating/cooling on/off	Read/write
HR 3	Operation mode	INT16	Operation mode	Read/write
HR 1	Leaving water main heating setpoint	INT16	Heating leaving water setpoint	Read/write
HR 10	DHW reheat setpoint	INT16	DHW setpoint	Read/write
IR 21	Unit error	INT16	Unit error	Read
IR 22	Unit error code	UINT16	Error code	Read
IR 42	Return water temperature	INT16	Return water temperature	Read
IR 43	Domestic hot water temperature	INT16	DHW temperature	Read
IR 44	Outside air temperature	INT16	Outside temperature	Read

Enable Modbus control & writes

No software enable. Choose the operating mode on SW1.1-SW1.2 and the address on SW1.3-SW1.8; the DCOM answers as a Modbus slave once it has synchronised with the Altherma master.

Communication loss & watchdog

If P1P2 communication with the Altherma master fails for 60 seconds the DCOM falls back to waiting for the master; after 3 more minutes it reports Timeout (unit error 1, error code U8). No Modbus-side watchdog is documented.

Byte order & addressing

Reads are limited to 64 registers; function 06 for single writes, function 16 for up to 64.
Setpoint writes outside the configured range are clamped to the nearest limit; other out-of-range writes are ignored.
Sensor temperatures are signed and divided by 100; setpoints are whole degrees.

Network setup

Power: regulated 15-24 VDC, at least 80 mA (polarity independent).
RS-485 is fixed at 9600 baud, 8 data bits, no parity, 1 stop bit.
Connect the RS-485 common terminal C to the other devices' common; termination is not needed at 9600 baud, and if you add it you must add failsafe bias too.
Up to 32 devices per segment, bus topology, under 1000 m.

Watch out for

With Altherma 2 and EKRUCBL / EKRUHML user interfaces the DCOM works only if no LAN adapter is connected.
Values read 32765 until synchronisation finishes; do not treat that as a temperature.

Daikin DCOM-LT/MB Modbus interface for Daikin Altherma LT heat pumps Modbus registers

Address = 0-based wire address (the documented number is shown under it when it differs) · Order: ABCD = high word first, CDAB = low word first · Scale: engineering value = raw × scale.

32 of 32 shown

Daikin DCOM-LT/MB Modbus interface for Daikin Altherma LT heat pumps Modbus register map
Table	Address	Name	Description	Type	Scale	Units	Access	Values / bits
IR	21	Unit error		INT16			Read	0 = No error; 1 = Fault; 2 = Warning
IR	22	Unit error code	Two ASCII characters, high byte first; 11565 = "--" (no error).	UINT16			Read	11565 = -- (no error); 21816 = U8 (DCOM timed out waiting for master)
IR	23	Unit error sub code	0-99 while an error exists; 32766 when there is none.	INT16			Read
IR	30	Circulation pump running		INT16			Read	0 = Off; 1 = On
IR	31	Compressor run		INT16			Read	0 = Off; 1 = On
IR	32	Booster heater run		INT16			Read	0 = Off; 1 = On
IR	33	Disinfection operation		INT16			Read	0 = Off; 1 = On
IR	35	Defrost/startup		INT16			Read	0 = Off; 1 = On
IR	36	Hot start		INT16			Read	0 = Off; 1 = On
IR	37	3-way valve	Which circuit the valve currently serves.	INT16			Read	0 = Space heating; 1 = DHW
IR	38	Operation mode (actual)		INT16			Read	1 = Heating; 2 = Cooling
IR	40	Leaving water temperature pre PHE	Before the plate heat exchanger.	INT16	× 0.01	°C	Read
IR	41	Leaving water temperature pre BUH	Before the backup heater.	INT16	× 0.01	°C	Read
IR	42	Return water temperature		INT16	× 0.01	°C	Read
IR	43	Domestic hot water temperature		INT16	× 0.01	°C	Read
IR	44	Outside air temperature		INT16	× 0.01	°C	Read
IR	45	Liquid refrigerant temperature		INT16	× 0.01	°C	Read
IR	49	Flow rate	Water flow; raw value is litres per minute x 100.	INT16	× 0.01	l/min	Read
IR	50	Remocon room temperature	Room temperature measured by the user interface.	INT16	× 0.01	°C	Read
HR	1	Leaving water main heating setpoint	Whole °C, 25-55 range; out-of-range writes are clamped.	INT16		°C	Read/write
HR	2	Leaving water main cooling setpoint	Whole °C, 5-22 range; out-of-range writes are clamped.	INT16		°C	Read/write
HR	3	Operation mode		INT16			Read/write	0 = Auto; 1 = Heating; 2 = Cooling
HR	4	Space heating/cooling on/off		INT16			Read/write	0 = Off; 1 = On
HR	6	Room thermostat heating setpoint	Whole °C, 12-30 range.	INT16		°C	Read/write
HR	7	Room thermostat cooling setpoint	Whole °C, 15-35 range.	INT16		°C	Read/write
HR	9	Quiet mode operation		INT16			Read/write	0 = Off; 1 = On
HR	10	DHW reheat setpoint	Domestic hot water target, whole °C, 30-60 range.	INT16		°C	Read/write
HR	12	DHW reheat on/off		INT16			Read/write	0 = Off; 1 = On
HR	13	DHW booster mode on/off		INT16			Read/write	0 = Off; 1 = On
HR	53	Weather dependent mode	How the leaving water setpoint is chosen.	INT16			Read/write	0 = Fixed; 1 = Weather dependent; 2 = Fixed + scheduled; 3 = Weather dependent + scheduled
HR	54	Weather dependent LWT heating setpoint offset	Shift of the heating curve, -10 to +10 °C.	INT16		°C	Read/write
HR	55	Weather dependent LWT cooling setpoint offset	Shift of the cooling curve, -10 to +10 °C.	INT16		°C	Read/write

Notes on this map

Three special values can come back from any register when read as 16-bit: 32767 = register unsupported, 32766 = not available in the current configuration, 32765 = value not loaded yet. Expect 32765 everywhere while the DCOM synchronises with the Altherma (up to 8 minutes after power-up).
The manual types the setpoints and offsets as plain int16 with no scaling, so they are whole degrees; only the sensor temperatures (temp16) are in hundredths.
The DCOM can also run in Sequencer mode (used with the DCOM-LT/IO variant) and Smart Grid mode, chosen with DIP switches SW1.1-SW1.2 (SW1.1-SW1.4 in Smart Grid mode). Sequencer mode moves several input registers (for example outside air to 133); those alternative registers are not listed here.
Which functions actually work depends on the Altherma model and its user interface; unsupported ones return 32767 or 32766.
The unit error code is two ASCII characters packed into one register: high byte = first character, low byte = second (for example 14152 = '7H'). The sub code is a separate number.

Check it against the live device. A register map tells you what the controller can expose; the unit in front of you may differ by firmware or options. Read a few registers and compare them with the unit’s own display; for a 32-bit value the float and byte-order decoder shows all four word orders at once, so a wrong word order or scale is obvious. New to this? Read what a register map is and why addresses are off by one.

Questions about the Daikin DCOM-LT/MB Modbus interface for Daikin Altherma LT heat pumps

What Modbus registers does the Daikin DCOM-LT/MB Modbus interface for Daikin Altherma LT heat pumps have?

This map lists 32 registers: 19 input registers, 13 holding registers. 13 of them can be written. Registers vary with firmware, options and configuration, so confirm against the device.

Are the Daikin DCOM-LT/MB Modbus interface for Daikin Altherma LT heat pumps Modbus addresses 0-based or 1-based?

The manufacturer's document numbers registers 0-based (the number printed is the address on the wire). Every address on this page is the 0-based wire address you put in the request; the documented number is shown under it where it differs. The manual numbers registers by offset with 'Register Base 0', so the printed offset is the wire address. Its own example: holding register 1 = Modicon 40002, input register 20 = Modicon 30021.

What are the minimum Modbus registers to integrate the Daikin DCOM-LT/MB Modbus interface for Daikin Altherma LT heat pumps?

Start with HR 4 Space heating/cooling on/off (Space heating/cooling on/off); HR 3 Operation mode (Operation mode); HR 1 Leaving water main heating setpoint (Heating leaving water setpoint); HR 10 DHW reheat setpoint (DHW setpoint); IR 21 Unit error (Unit error); IR 22 Unit error code (Error code); IR 42 Return water temperature (Return water temperature); IR 43 Domestic hot water temperature (DHW temperature); IR 44 Outside air temperature (Outside temperature).

How do I enable Modbus control of the Daikin DCOM-LT/MB Modbus interface for Daikin Altherma LT heat pumps?

No software enable. Choose the operating mode on SW1.1-SW1.2 and the address on SW1.3-SW1.8; the DCOM answers as a Modbus slave once it has synchronised with the Altherma master.

What happens when the Daikin DCOM-LT/MB Modbus interface for Daikin Altherma LT heat pumps loses Modbus communication?

Where is the official Daikin DCOM-LT/MB Modbus interface for Daikin Altherma LT heat pumps Modbus documentation?

The source is DCOM-LT/MB Reference Manual (English) (534-252-A3), published free by Daikin at https://www.daikin.eu/content/dam/document-library/installation-manuals/ctrl/bms/dcom-lt-mb/DCOM-LT-MB_reference%20and%20installation%20manual_English.pdf.

Source: DCOM-LT/MB Reference Manual (English) · 534-252-A3. Register numbers, types, scaling and units are reproduced as published; descriptions are our own.