Vaisala GMP251 CO2 probe

Register map(s) for this device, each at the trust rung it earned. Addresses are 0-based as on the wire. Word order and scaling are confirmed at the hardware-verified rung and at no rung below it.

interop-verified

Read + decoded cleanly over a real network vs an independent third-party Modbus test server (transport and framing; reads are remapped into the server's register window, so the map's own addresses are NOT exercised). Does NOT prove word order/scaling against the actual device.

point	address (0-based)	type	scale	unit
measured_co2_value_float	0	F32LE	1	ppm
compensation_t	2	F32LE	1	°C
measured_t	4	F32LE	1	°C
measured_co2_value_i16	256	I16	1	ppm
measured_co2_value_i16_scaled	257	I16	10	ppm
power_up_pressure_compensation	512	F32LE	1	hPa
power_up_temperature_compensation	514	F32LE	1	°C
power_up_humidity_compensation	516	F32LE	1	%RH
power_up_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
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.
interop-verified — Read + decoded cleanly over a real network vs an independent third-party Modbus test server (transport and framing; reads are remapped into the server's register window, so the map's own addresses are NOT exercised). Does NOT prove word order/scaling against the actual device.
simulator-verified — Decoded correctly against a protocol emulator seeded with known-true values — catches word-order/scaling errors vs the emulator, not the physical device.
hardware-verified x1 — Matched a real device with operator-attested readings as ground truth — the only rung that confirms word order + scaling on the wire.

If your device isn't here, upload its register map. Generation is free, and the connector keeps the rung it earns under testing.

To prove a connector against your own device, order hardware verification. If it doesn't verify, you don't pay.

Platform builders can license the verified library: machine-readable maps, the firmware matrix, API access.