Hi DonManfred...
Modbus RTU is an open serial protocol derived from the Master/Slave architecture originally developed by Modicon (now Schneider Electric). It is a widely accepted serial level protocol due to its ease of use and reliability. Modbus RTU is widely used within Building Management Systems (BMS) and Industrial Automation Systems (IAS).
Modbus RTU messages are a simple 16-bit structure with a CRC (Cyclic-Redundant Checksum). The simplicity of these messages is to ensure reliability. Due to this simplicity, the basic 16-bit Modbus RTU register structure can be used to pack in floating point, tables, ASCII text, queues, and other unrelated data.
This protocol primarily uses an RS-232 or RS-485 serial interfaces for communications and is supported by almost every commercial SCADA, HMI, OPC Server and data acquisition software program in the marketplace. This makes it very easy to integrate Modbus compatible equipment into new or existing monitoring and control applications.
(copas from
https://www.rtautomation.com/technologies/modbus-rtu/)
I found discussion about this library that maybe can be wrapped here :
https://stackoverflow.com/questions/37459012/android-modbus-rtu-library