Android Code Snippet ✅ Modbus CRC-16 / CRC16 checksum function

SubName: Modbus CRC-16 is used to ensure data integrity in Modbus RTU (Remote Terminal Unit) communications.
Description: This function takes a command message Bytes and performs the necessary bitwise operations to generate a 16-bit CRC checksum, and outputs this checksum as a hexadecimal string.

The following code calculates the Modbus RTU CRC-16 (LSB-first / little-endian), the standard CRC (Cyclic Redundancy Check) used for Modbus frames.

Let's use 010107DE000A as an example to calculated it's Modbus CRC-16 value, which is 43DD.

    Dim BC As ByteConverter
    Dim Data() As Byte = BC.HexToBytes("010107DE000A")
    Log(CalcModbusCRC(Data))

Sub CalcModbusCRC(Bytes() As Byte) As String
    Dim CRCr As Int = 0xFFFF
    For i = 0 To Bytes.Length - 1
        CRCr = Bit.Xor(CRCr, Bit.And(Bytes(i), 0xFF))
        For j = 0 To 7
            If (Bit.And(CRCr, 1) = 1) Then
                CRCr = Bit.Xor(Bit.ShiftRight(CRCr, 1), 0xA001)
            Else
                CRCr = Bit.ShiftRight(CRCr, 1)
            End If
        Next
    Next
 
    'Low byte first (Modbus standard)
    Dim low As Int = Bit.And(CRCr, 0xFF)
    Dim high As Int = Bit.And(Bit.ShiftRight(CRCr, 8), 0xFF)

    Return Bit.ToHexString(high).ToUpperCase & Bit.ToHexString(low).ToUpperCase
End Sub

The full Modbus RTU frame that would be transmitted is 01 01 07 DE 00 0A 43 DD.

Here is the byte by byte Breakdown

Hex Value	Description
01	Slave Address (Unit ID): The address of the device being targeted.
01	Function Code: Read Coils (Discrete Outputs).
07DE	Starting Address: The address of the first coil to read (7DE hex = 2014 decimal).
000A	Quantity: The number of coils to read (0A hex = 10 decimal).
43	CRC Low Byte: The Least Significant Byte of the CRC.
DD	CRC High Byte: The Most Significant Byte of the CRC.

Enjoy...