Greystone CHTDT CO2/RH/T sensor

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
co2	0	U16	1
relative_humidity	1	U16	0.1
temperature_c	2	I16	0.1
temperature_f	3	I16	0.1

link settings as documented (unverified): RTU; default 9600 8N1; unit ID 1; CRC polynomial is configurable (A001, 1021, 8005, 8408). Slave response delay is configurable (MI, 50, 100, 150, 200, 250, 300, 350ms).

⚠ Do not use in an explosive or hazardous environment, with combustible or flammable gases, as a safety or emergency stop device or in any other application where failure of the product could result in personal injury. — Starkest reads only; we never write a control register.

bench facts as documented (unverified)

terminals	Screw terminal block (14 to 22 AWG)
A/B polarity	A(-), B(+)
termination	121 ohm termination resistor selectable via jumper
connector	Screw terminal block
isolation	Relay output is completely isolated
shield/ground	The ground wire of the shielded pair should be connected to earth ground at the end of the network and the master is not grounded.
wiring notes	Use 22 AWG shielded wiring for all connections
protocol notes	Slave response delay is configurable from MI (minimum, >3.5 character times) to 350ms.
identification notes	On start-up, the LCD will indicate the software version number, the slave address and the baud rate.
power	20 - 28 Vac/dc (non-isolated half-wave rectified), 120 mA max @ 24 Vdc, 212 mA max @ 24 Vac
environment	Operating Conditions: 0 to 50°C (32 to 122°F), 0-95 %RH non-condensing. Storage Temperature: -20 to 60°C (-4 to 140°F). Enclosure: Polycarbonate, UL94-V0, IP65 (NEMA 4X)
accuracy	CO2: ± (30ppm + 3% of measured value). RH: ±2 %RH (5 to 95 %RH). Temp: ±0.2°C
certifications	CE
doc revision	IN-GE-CHTDTXXXMOD-04

⚠ disputed: register_map: co2 @ 0, relative_humidity @ 1, temperature_c @ 2, temperature_f @ 3 — disputed by CHT Series Modbus Setup Guide (SG-CHTXXXMOD-02) which maps Relay/Unit Status @ 0 (40001), CO2 @ 1 (40002), RH @ 2 (40003), and Temperature @ 3 (40004)

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

Depending on the firmware version of the CHT Series Modbus Setup Guide (e.g., SG-CHTXXXMod-01-01 vs SG-CHTXXXMOD-02), the register map offsets differ. In version 01-01, register 40001 is Relay Status, 40002 is CO2, 40003 is Temperature, and 40004 is RH. In version 02, register 40001 is Unit Status, 40002 is CO2, 40003 is RH, and 40004 is Temperature. This swap of RH and Temperature registers can cause integration issues if the wrong guide version is used. [source]
confirmed: comms.baud_rates: [9600, 19200, 38400, 57600, 76800, 115200] — confirmed by CHT Series Modbus Setup Guide (SG-CHTXXXMOD-02)
confirmed: comms.default_baud: 9600 — confirmed by CHTDT Modbus Series Installation Instructions (IN-GE-CHTDTXXXMOD-04)
confirmed: comms.default_unit_id: 1 — confirmed by CHTDT Modbus Series Installation Instructions (IN-GE-CHTDTXXXMOD-04)
confirmed: comms.notes: CRC polynomial is configurable (A001, 1021, 8005, 8408) and Slave response delay is configurable (MI, 50, 100, 150, 200, 250, 300, 350ms) — confirmed by CHTDT Modbus Series Installation Instructions (IN-GE-CHTDTXXXMOD-04)
confirmed: facts.accuracy: CO2: ± (30ppm + 3% of measured value). RH: ±2 %RH (5 to 95 %RH). Temp: ±0.2°C — confirmed by CHTDT Series Datasheet
confirmed: facts.doc_revision: IN-GE-CHTDTXXXMOD-04 — confirmed by CHTDT Modbus Series Installation Instructions
confirmed: physical.polarity: A(-), B(+) — confirmed by CHTDT Modbus Series Installation Instructions (IN-GE-CHTDTXXXMOD-04)

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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