QDW90A Pressure Transmitter

Modbus RTU

Submitted by

Last updated: 06. 2026

Requires external gateway
TapHome Multi-Protocol Gateway — product page

The QDW90A is a generic piezoresistive pressure and level transmitter manufactured by Qidian (Anhui Qidian Automation Technology Co., Ltd.). It communicates via Modbus RTU over RS-485 and connects to TapHome through the Multi-Protocol Gateway. The same hardware platform supports pressure, level, temperature, and pH measurement depending on the installed sensor element.

The template reads the measurement value from holding register H:4 with automatic decimal scaling based on H:3. Four service attributes display the current device configuration (slave address, baud rate, measurement unit, and decimal format), and one service action allows remote slave address changes.

Hardware connection

RS-485 wiring

Connect the QDW90A to the TapHome Multi-Protocol Gateway using the RS-485 terminals:

Sensor wire	Connection	Description
RED	+24 V	Power supply positive
BLACK	GND	Power supply ground
BLUE	A (RS485+)	Modbus data line A
YELLOW	B (RS485-)	Modbus data line B

Wire colors may vary between OEM variants. Always verify the wiring against the documentation included with your specific unit before connecting.

Communication parameters

Parameter	Default value
Baud rate	9600
Parity	None
Data bits	8
Stop bits	1
Slave ID	1

The baud rate is configurable from 1200 to 115200 via register H:1. The slave address range is 1–255 and can be changed remotely using the Rewrite the Slave Address service action in TapHome.

After changing the slave address or baud rate, the transmitter replies using the original settings before switching to the new values. You must also write H:15 = 0 to persist changes to non-volatile memory.

Device capabilities

Pressure/level measurement

The template exposes one device — Pressure Level — that reads the measurement value from holding register H:4 (Int16). The raw value is automatically divided by 10 raised to the power of the decimal point setting from H:3:

1
actual_value = raw_value / 10^(decimal_points)

The measurement unit depends on the sensor configuration stored in H:2. The transmitter supports 23 different units:

Code	Unit	Category
0	MPa	Pressure
1	KPa	Pressure
2	Pa	Pressure
3	bar	Pressure
4	mbar	Pressure
5	Kgcm2	Pressure
6	PSI	Pressure
7	mH2O	Pressure
8	mmH2O	Pressure
9	inH2O	Pressure
10	H2O	Pressure
11	mHg	Pressure
12	mmHg	Pressure
13	inHg	Pressure
14	atm	Pressure
15	Torr	Pressure
16	m	Level
17	cm	Level
18	mm	Level
19	Kg	Weight
20	°C	Temperature
21	pH	pH value
22	°F	Temperature

The template script applies an additional conversion for meters (code 16) and centimeters (code 17) to handle their specific scaling requirements.

Service attributes

The module exposes four service attributes for monitoring the device configuration:

Slave Address (H:0) — current Modbus slave address (1–255)
Baud Rate (H:1) — decoded baud rate from the internal code (1200, 2400, 4800, 9600, 19200, 38400, 57600, or 115200)
Unit (H:2) — currently configured measurement unit displayed as its abbreviation (MPa, KPa, bar, PSI, mH2O, °C, pH, etc.)
Decimal Points (H:3) — decimal point format (####, ###.#, ##.##, #.###, or .####)

Service action

Rewrite the Slave Address — writes a new slave address (1–255) to H:0 and saves to non-volatile memory by writing H:15 = 0. The transmitter responds using the original address before switching.

Additional capabilities

The QDW90A also provides a Float32 measurement value at registers H:22–H:23 (IEEE 754, big-endian ABCD byte order) which gives the direct reading without decimal scaling. Additional registers include the range zero point (H:5), range full point (H:6), zero offset calibration (H:12), serial parity configuration (H:37), and a factory reset command (H:16). These can be added as custom Modbus devices in TapHome if needed.

Available devices

QDW90A Pressure Transmitter Module

Service Attributes

Slave Address	Current Modbus slave address (1-255, default 1)
Baud Rate	Current baud rate decoded from internal code (1200-115200)
Unit	Currently configured measurement unit (MPa, KPa, Pa, bar, mbar, PSI, mH2O, mmH2O, °C, °F, pH, etc.)
Decimal Points	Decimal point display format (####, ###.#, ##.##, #.###, .####)

Service Actions

Rewrite the Slave Address

QDW90A Pressure transmitter - in

Service Attributes

Slave Address

modbusr(H, 0, int16);

Baud Rate

SWITCH(modbusr(H, 1, int16),
	0, 1200,
	1, 2400,
	2, 4800,
	3, 9600,
	4, 19200,
	5, 38400,
	6, 57600,
	7, 115200,
      NaN
);

Unit

switch(modbusr(H, 2, int16),
	0, "MPa",
	1, "KPa",
	2, "Pa",
	3, "bar",
	4, "mbar",
	5, "Kgcm2",
	6, "PSI",
	7, "mH2O",
	8, "mmH2O",
	9, "inH2O",
	10, "H2O",
	11, "mHg",
	12, "mmHg",
	13, "inHg",
	14, "atm",
	15, "Torr",
	16, "m",
	17, "cm",
	18, "mm",
	19, "Kg",
	20, "°C",
	21, "PH",
	22, "°F",
	NaN
);

Decimal points

SWITCH(modbusr(H, 3, int16),
	0, "####",
	1, "###.#",
	2, "##.##",
	3, "#.###",	
	4, "#.###",
	NaN
);

Service Actions

Rewrite the Slave Address

Parameters: Slave Address (1–255 SlaveAddr)

modbusw(SH, 0, int16, SlaveAddr);
modbusw(SH, 15, int16, 0);  # Save to user area

Pressure Level Variable Read-only

Measurement value with dynamic decimal scaling — reads H:4 divided by 10^(H:3) with unit-specific conversion for m and cm

Connection: Modbus RTU • 9600 baud• 8N1 • Slave ID: $[SlaveId]

Possible improvements (7)

H:22-23 Float Measurement Value — Float32 IEEE 754, big-endian ABCD byte order — direct measurement without decimal scaling. Alternative to H:4 integer method used by template.
H:5 Range Zero Point — Transmitter zero calibration value — useful for diagnostics
H:6 Range Full Point — Transmitter full scale calibration value — useful for diagnostics
H:12 Zero Offset — Int16 R/W, factory default 0. PV output = calibration value + offset. Writable via FC 0x06.
H:37 Serial Parity — Int16 R/W; 0=None, 1=Odd, 2=Even. Configures RS485 serial parity.
H:1 Baud Rate (write) — Int16 writable; code 0-7 to change baud rate. Readable as service attribute but no write action in template.
H:16 Factory Reset — Write 1 to restore factory parameters. Caution: reverts address, baud rate, and calibration data.

Download Template

Sources

QDW90A Modbus RTU Communication Protocol
s2ins.com 2026-04-19
PDF
Home Assistant Community — QDW90A Modbus Integration
community.home-assistant.io 2026-04-19

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