Vaisala GMP251 CO2 probe

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
Measured CO2 value	0	F32LE	1	ppm
Compensation T	2	F32LE	1	°C
Measured T	4	F32LE	1	°C
Measured CO2 value	256	I16	1	ppm
Measured CO2 value	257	I16	10	ppm
Power-up value for pressure compensation	512	F32LE	1	hPa
Power-up value for temperature compensation	514	F32LE	1	°C
Power-up value for humidity compensation	516	F32LE	1	%RH
Power-up value for oxygen compensation	518	F32LE	1	%O2
Volatile pressure compensation	520	F32LE	1	hPa
Volatile temperature compensation	522	F32LE	1	°C
Volatile humidity compensation	524	F32LE	1	%RH
Volatile oxygen compensation	526	F32LE	1	%O2
Modbus address	768	U16	1
Serial speed	769	U16	1
Serial parity	770	U16	1
Serial stop bits	771	U16	1
Pressure compensation mode	772	U16	1
Temperature compensation mode	773	U16	1
Humidity compensation mode	774	U16	1
Oxygen compensation mode	775	U16	1
CO2 filtering factor	776	U16	1
Device status	2048	U16	1
CO2 status	2049	U16	1

link settings as documented (unverified): RTU; default 19200 8N2; unit ID 240; FC 03,16,43

bench facts as documented (unverified)

connector	M12 5-pin male
isolation	non-isolated 2-wire interface
wiring notes	The data lines are RS-485 D- and RS-485 D+. Ground is shared with power supply.
sentinel values	NaN for floating point data or 0x0000 for integer data
FC 43 device ID	supported
power	12 ... 30 VDC with digital output. Typical average power consumption <0.4 W, maximum 0.5 W.
environment	Operating temperature of CO2 measurement: -40 ... +60 °C. Storage temperature: -40 ... +70 °C. Pressure: Compensated 500 ... 1100 hPa, Operating < 1.5 bar.
doc revision	M211799EN-E, August 2016

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

confirmed: baud_rates: [4800, 9600, 19200, 38400, 57600, 115200] — confirmed by Vaisala GMP251 User Guide (M211799EN-G)
confirmed: default_baud: 19200 — confirmed by Vaisala GMP251 User Guide (M211799EN-G)
confirmed: default_unit_id: 240 — confirmed by Vaisala GMP251 User Guide (M211799EN-G)
confirmed: framing: ['RTU'] — confirmed by Vaisala GMP251 User Guide (M211799EN-G)
confirmed: function_codes: [3, 16, 43] — confirmed by Vaisala GMP251 User Guide (M211799EN-E)
confirmed: parity: 'N' — confirmed by Vaisala GMP251 User Guide (M211799EN-G)
confirmed: stop_bits: 2 — confirmed by Vaisala GMP251 User Guide (M211799EN-G)
confirmed: doc_revision: 'M211799EN-E, August 2016' — confirmed by Vaisala GMP251 User Guide (M211799EN-G) revision history

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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