Mitsubishi ATW heat pump (needs MelcoBEMS MINI A1M)

Register map(s) for this device, with a note of how far each has been checked. Addresses are 0-based, as on the wire. Scale and byte order are confirmed only at the hardware-verified rung.

coherence-checked

Round-trips through a real Modbus stack with no overlapping addresses and a word-order-sensitive codec — the map is internally coherent and wire-decodable. Does NOT prove byte-order or scaling match a real device: the emulator is seeded from the map's own types, so a uniformly wrong byte-order still round-trips. Only hardware-verified confirms byte-order/scale on the wire.

point	address (0-based)	type	scale	unit
system_on_off	25	U16	1
operating_mode	26	U16	1
set_tank_water_temperature_signed	30	I16	0.01	°C
set_tank_water_temperature	31	U16	0.01	°C
hc_thermostat_target_temperature_zone_1_signed	32	I16	0.01	°C
hc_thermostat_target_temperature_zone_1	33	U16	0.01	°C
hc_thermostat_target_temperature_zone_2_signed	34	I16	0.01	°C
hc_thermostat_target_temperature_zone_2	35	U16	0.01	°C
outdoor_temperature_by_bms_signed	50	I16	0.1	°C
outdoor_temperature_by_bms	51	U16	0.1	°C
setting_water_temperature_signed	52	I16	0.01	°C
setting_water_temperature	53	U16	0.01	°C
thermostat_target_temperature_zone_1_signed	54	I16	0.01	°C
thermostat_target_temperature_zone_1	55	U16	0.01	°C
thermostat_target_temperature_zone_2_signed	56	I16	0.01	°C
thermostat_target_temperature_zone_2	57	U16	0.01	°C
heat_pump_frequency_master	73	U16	1	Hz
temperature_setpoint_zone_1_signed	81	I16	0.01	°C
temperature_setpoint_zone_1	82	U16	0.01	°C
temperature_setpoint_zone_2_signed	83	I16	0.01	°C
temperature_setpoint_zone_2	84	U16	0.01	°C
flow_temperature_setpoint_zone_1_signed	85	I16	0.01	°C
flow_temperature_setpoint_zone_1	86	U16	0.01	°C
flow_temperature_setpoint_zone_2_signed	87	I16	0.01	°C
flow_temperature_setpoint_zone_2	88	U16	0.01	°C
room_temperature_zone_1_signed	93	I16	0.01	°C
room_temperature_zone_1	94	U16	0.01	°C
room_temperature_zone_2_signed	95	I16	0.01	°C
room_temperature_zone_2	96	U16	0.01	°C
refrigerant_liquid_temperature_signed	97	I16	0.01	°C
refrigerant_liquid_temperature	98	U16	0.01	°C
outdoor_ambient_temperature_signed	99	I16	0.1	°C
outdoor_ambient_temperature	100	U16	0.1	°C
flow_temperature_signed	101	I16	0.01	°C
flow_temperature	102	U16	0.01	°C
return_temperature_signed	103	I16	0.01	°C
return_temperature	104	U16	0.01	°C
tank_water_temperature_signed	105	I16	0.01	°C
tank_water_temperature	106	U16	0.01	°C
boiler_flow_temperature_signed	115	I16	0.01
boiler_flow_temperature	116	U16	0.01
boiler_return_temperature_signed	117	I16	0.01
boiler_return_temperature	118	U16	0.01
evaporating_temperature_signed	230	I16	0.01
evaporating_temperature	231	U16	0.01
condensing_temperature_signed	232	I16	0.01
condensing_temperature	233	U16	0.01
flow_rate	299	U16	1

link settings as documented (unverified): baud 1200/2400/4800/9600/14400/19200/28800/38400/56000/57600/115200; FC 01,02,03,04,05,06,15,16

bench facts as documented (unverified)

terminals	RS-485
firmware register	Holding 10, Input 3
doc revision	1.0.4
firmware applicability	3.0.25

field reports — community-sourced, unverified (checked 2026-06-11)

In firmware versions prior to 3.0.16, writes to Modbus register 40027 (operating_mode @ 26) were not performed correctly for values greater than 7. [source]
In firmware versions prior to 3.0.1, the 'Outdoor Temperature By BMS' Modbus register (Holding 51 / 40052) did not report the correct value. [source]
In firmware versions prior to 3.0.15, the Modbus comms counter register (Holding 11 / 40012) did not increment correctly. [source]
In document version 1.0.5, Modbus addresses 40053 & 40054 (setting_water_temperature_signed @ 52 and setting_water_temperature @ 53) were changed to [READ ONLY]. [source]
For the A1M+ (Ethernet variant), the factory default IP address (192.168.92.114) is only reachable after power-cycling the device by disconnecting the CN105 connector from the heat pump. [source]
confirmed: comms.baud_rates: [1200,2400,4800,9600,14400,19200,28800,38400,56000,57600,115200] — confirmed by Procon MelcoBEMS MINI (A1M) Installation & User Manual
confirmed: comms.function_codes: [1,2,3,4,5,6,15,16] — confirmed by Procon MelcoBEMS MINI (A1M) Installation & User Manual
confirmed: facts.doc_revision: 1.0.4 — confirmed by Procon MelcoBEMS MINI (A1M) ATW Modbus Register Tables V1.0.4
confirmed: facts.firmware_applicability: 3.0.25 — confirmed by Procon MelcoBEMS MINI (A1M) ATW Modbus Register Tables V1.0.4
confirmed: facts.identification: firmware_register Holding 10, Input 3 — confirmed by Procon MelcoBEMS MINI (A1M) Installation & User Manual
confirmed: facts.physical: terminals RS-485 — confirmed by Procon MelcoBEMS MINI (A1M) Installation & User Manual
confirmed: register_map: system_on_off @ 25, operating_mode @ 26, set_tank_water_temperature_signed @ 30, set_tank_water_temperature @ 31, hc_thermostat_target_temperature_zone_1_signed @ 32, hc_thermostat_target_temperature_zone_1 @ 33, hc_thermostat_target_temperature_zone_2_signed @ 34, hc_thermostat_target_temperature_zone_2 @ 35, outdoor_temperature_by_bms_signed @ 50, outdoor_temperature_by_bms @ 51, setting_water_temperature_signed @ 52, setting_water_temperature @ 53, thermostat_target_temperature_zone_1_signed @ 54, thermostat_target_temperature_zone_1 @ 55, thermostat_target_temperature_zone_2_signed @ 56, thermostat_target_temperature_zone_2 @ 57, heat_pump_frequency_master @ 73, temperature_setpoint_zone_1_signed @ 81, temperature_setpoint_zone_1 @ 82, temperature_setpoint_zone_2_signed @ 83, temperature_setpoint_zone_2 @ 84, flow_temperature_setpoint_zone_1_signed @ 85, flow_temperature_setpoint_zone_1 @ 86, flow_temperature_setpoint_zone_2_signed @ 87, flow_temperature_setpoint_zone_2 @ 88, room_temperature_zone_1_signed @ 93, room_temperature_zone_1 @ 94, room_temperature_zone_2_signed @ 95, room_temperature_zone_2 @ 96, refrigerant_liquid_temperature_signed @ 97, refrigerant_liquid_temperature @ 98, outdoor_ambient_temperature_signed @ 99, outdoor_ambient_temperature @ 100, flow_temperature_signed @ 101, flow_temperature @ 102, return_temperature_signed @ 103, return_temperature @ 104, tank_water_temperature_signed @ 105, tank_water_temperature @ 106, boiler_flow_temperature_signed @ 115 — confirmed by Procon MelcoBEMS MINI (A1M) ATW Modbus Register Tables V1.0.4

The trust ladder

Each rung states what it proved and what it did not. Gray until proven; green is earned by hardware only.

generated — A prompt produced it. Unproven — anyone can regenerate this.
structure-verified — Decode path is structurally sound (addresses, widths, finite values). Does NOT prove word order or scaling.
coherence-checked — Round-trips through a real Modbus stack with no overlapping addresses and a word-order-sensitive codec — the map is internally coherent and wire-decodable. Does NOT prove byte-order or scaling match a real device: the emulator is seeded from the map's own types, so a uniformly wrong byte-order still round-trips. Only hardware-verified confirms byte-order/scale on the wire.
hardware-verified (software-key) x1 — Matched a real device with operator-attested readings as ground truth — the only rung that confirms word order + scaling on the wire.

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