Many Chinese sensor manufacturers (like XY) host their manuals on public web directories. Try the following URL pattern (replace X with the latest version): http://www.xy-sensor.com/down/XY-MD02_manual.pdf Note: If this link is broken, use Method 2.
If you have multiple XY-MD02 sensors on one RS485 bus, each needs a unique Device ID (default is 0x01 or 1). The manual explains how to use (Write Single Register). xy-md02 manual pdf