Modbus RTU Frame Builder and CRC-16 Calculator - Code and Compile Wiki

A Modbus RTU frame is the slave address, the function code, the data, and a 2-byte CRC-16. Get one byte wrong and the slave stays silent. Use the builder to create a correct request, or paste a frame from a serial sniffer to check its CRC.

How the frame is built

Read holding registers (function 03), slave 1, address 0, 10 registers:

01        slave address
03        function code
00 00     start address (0-based)
00 0A     number of registers
C5 CD     CRC-16, low byte first

The CRC-16/MODBUS starts at FFFF, uses the polynomial A001 (reversed 8005), and is sent low byte first.
Addresses are 0-based on the wire. The holding register that a manual calls 40001 is address 0 in the frame, 40010 is address 9.
Modbus TCP has no CRC. It puts a 7-byte MBAP header in front instead: transaction ID, protocol ID 0000, length, and the unit ID.

Modbus float converter

Turn the two registers you read into a float in all four byte orders.

Bit and number converter

Decode coils, status words and register values bit by bit.