The sensor does not answer. What should I check?

Check the wiring first: A+ to A and B- to B; swapped lines are the most common cause. Then the settings: address 1, 9600 baud, 8N1 out of the box. Temperature and Humidity are input registers, so read them with FC 04; reading 0x0001 with FC 03 is not documented and may return an error.

Below freezing the temperature shows over 6500 °C.

Temperature is a signed 16-bit value with scale 0.1. Read as unsigned, -20.5 °C (0xFF33) becomes 6533.1. This template already uses int16; check that a custom copy did not change it to uint16.

Does the Baud Rate register hold a code or the baud rate itself?

The listing shows both. The register table says 0 = 9600, 1 = 14400 and 2 = 19200, while the FC 16 example writes 0x2580 (9600) into the same register. Read Baud Rate while the sensor runs at 9600: 0 means it uses codes, 9600 means it holds the rate. Write the new value in that same form, then switch the bus to the new speed.

How do I write the address or a correction value?

Write with FC 06 to one register, or with FC 16 to several. The example 01 06 01 01 00 08 from the listing sets the address to 8; the sensor then answers on address 8 only. Temperature Correction and Humidity Correction are tenths from -10.0 to 10.0, so a raw value of 20 sets 2.0 °C and -51 sets -5.1 %RH.

Readings drop out with several sensors on one cable.

Every sensor needs its own address; two sensors on address 1 corrupt each other's answers. On a long line, terminate both ends of the bus with 120 ohm and keep the supply close to the sensors. ModbusCloud reads every 5 minutes by default and at most once a minute, so the bus load from these sensors stays low.

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Modbus RTU

Generic XY-MD02 RS485 Temperature and Humidity Sensor

Name: Generic XY-MD02 RS485 Temperature and Humidity Sensor
Brand: Generic
SKU: 0b8d53c1-1e34-402e-b656-9ae09931b4eb

Generic

XY-MD02 RS485 sensor: temperature, humidity, address, baud rate and correction over Modbus RTU.

Overview

The XY-MD02 is an RS485 temperature and humidity sensor in a white vented housing with a four-pole screw terminal: B- and A+ for the RS485 bus, minus and plus for 5 to 30 V DC. The housing carries only the model name and no brand, so this template is listed under Generic. Temperature and relative humidity are input registers, read with FC 04 at 0x0001 and 0x0002, both ten times the actual value; temperature is signed, so values below 0 °C read correctly. The settings are holding registers: device address, baud rate, and a correction value for temperature and one for humidity, each from -10.0 to 10.0. Default connection: address 1, 9600 baud, 8 data bits, no parity, 1 stop bit.

Key readings

Temperature in °C (0.1 resolution, negative values supported)
Relative humidity in %RH (0.1 resolution)
Device address (default 1)
Baud rate (default 9600)
Temperature and humidity correction (-10.0 to 10.0)

Available commands

Device address (FC 06 to 0x0101)
Baud rate (FC 06 to 0x0102)
Temperature correction (-10.0 to 10.0 °C)
Humidity correction (-10.0 to 10.0 %RH)

Use cases

Log Temperature and Humidity in a server room, cold store or warehouse and keep 365 days of history per sensor.
Set a threshold alarm on Humidity for a greenhouse, archive or crawl space so you get an email when it passes your limit.
Watch Temperature below 0 °C in an outdoor cabinet or freezer room; the signed register reads frost correctly.
Align a sensor that reads off against a reference by writing Temperature Correction or Humidity Correction.
Check Device Address and Baud Rate remotely before you add more sensors to the same RS485 line.

Installation

1Wire the sensor: A+ to RS485 A and B- to RS485 B of the ModbusCloud Gateway, plus and minus to a 5 to 30 V DC supply. Fit a 120 ohm terminating resistor at the end of a long line.
2Give every sensor on the bus its own address before you connect them together. The default is 1; write a new value to Device Address with FC 06.
3In ModbusCloud, add a Modbus device (RTU) with address 1, 9600 baud, 8 data bits, no parity and 1 stop bit, and import this template.
4Check the readings: Temperature and Humidity should match the room, and Device Address should read the address you set.
5Read Baud Rate once before you change it: 0 means the code table applies, 9600 means the register holds the baud rate itself.

Things to watch for

The sensor does not answer. What should I check?: Check the wiring first: A+ to A and B- to B; swapped lines are the most common cause. Then the settings: address 1, 9600 baud, 8N1 out of the box. Temperature and Humidity are input registers, so read them with FC 04; reading 0x0001 with FC 03 is not documented and may return an error.
Below freezing the temperature shows over 6500 °C.: Temperature is a signed 16-bit value with scale 0.1. Read as unsigned, -20.5 °C (0xFF33) becomes 6533.1. This template already uses int16; check that a custom copy did not change it to uint16.
Does the Baud Rate register hold a code or the baud rate itself?: The listing shows both. The register table says 0 = 9600, 1 = 14400 and 2 = 19200, while the FC 16 example writes 0x2580 (9600) into the same register. Read Baud Rate while the sensor runs at 9600: 0 means it uses codes, 9600 means it holds the rate. Write the new value in that same form, then switch the bus to the new speed.
How do I write the address or a correction value?: Write with FC 06 to one register, or with FC 16 to several. The example 01 06 01 01 00 08 from the listing sets the address to 8; the sensor then answers on address 8 only. Temperature Correction and Humidity Correction are tenths from -10.0 to 10.0, so a raw value of 20 sets 2.0 °C and -51 sets -5.1 %RH.
Readings drop out with several sensors on one cable.: Every sensor needs its own address; two sensors on address 1 corrupt each other's answers. On a long line, terminate both ends of the bus with 120 ohm and keep the supply close to the sensors. ModbusCloud reads every 5 minutes by default and at most once a minute, so the bus load from these sensors stays low.

Registers (6)

6 read / 4 write

Name	Address	Function	Type	Unit	Access
Temperature	•••	•••	•••	°C	R
Humidity	•••	•••	•••	%RH	R
Device Address	•••	•••	•••		R/W
Baud Rate	•••	•••	•••		R/W
Temperature Correction	•••	•••	•••	°C	R/W
Humidity Correction	•••	•••	•••	%RH	R/W

Loading registers…

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