SQL Function to Swap Words for 32-Bit Float Modbus Data

You can convert the original value to BINARY(4) and then use the SUBSTRING() function to extract and swap the first-two and last-two bytes - SUBSTRING(BinaryValue, 3, 2) + SUBSTRING(BinaryValue, 1, 2).

The trick is converting back to a REAL (32-bit floating point) value. SQL Server has no built-in capability to convert BINARY to REAL. Luckily, that need has arisen before in the question Convert float to varbinary and back to float, and we have a function for that.

CREATE FUNCTION ConvertBinaryToReal(@Binary VARBINARY(MAX)) RETURNS REAL
AS
BEGIN
    -- IEEE 754 Single Precision binary to float
    -- Layout:
    --   32 bits total
    --   1 bit sign
    --   8 bits exponent (excess 127)
    --   23 bits fractional mantissa  (with implicit leading 1) = 1.xxxxx
    -- Does not support IEEE NaN, +Inf, or -Inf values. 

    IF (@Binary IS NULL OR DATALENGTH(@Binary) <> 4) RETURN NULL
    IF (@Binary = 0x00000000) RETURN 0
    IF (@Binary = 0x80000000) RETURN -0e0 -- IEEE Negative 0

    DECLARE @Int32 INT = CAST(@Binary AS INT)
    DECLARE @One REAL = 1
    DECLARE @Two REAL = 2
    DECLARE @Mantissa REAL = @One + (@Int32 & 0x007FFFFF) * POWER(@Two, -23)
    DECLARE @Exponent INT = (@Int32 & 0x7f800000) / 0x00800000 - 127

    IF (@Exponent = 128) RETURN NULL -- Unsupported special: Inf, -Inf, NaN

    RETURN SIGN(@Int32) * @Mantissa * POWER(@Two, @Exponent)
END

Using your sample value 409.9535, the binary representation is 0x43CCFA0C, and the swapped binary representation is 0xFA0C43CC.

The following will swap the bytes and convert the result to a REAL value.

SELECT *
FROM (
    VALUES (0xFA0C43CC)
) SB(SwappedBinary)
CROSS APPLY (
    SELECT SUBSTRING(SB.SwappedBinary, 3, 2)
           + SUBSTRING(SB.SwappedBinary, 1, 2)
           AS OriginalBinary
) OB
CROSS APPLY (
    SELECT dbo.ConvertBinaryToReal(OB.OriginalBinary) AS OriginalValue
) OV

Results:

See this db<>fiddle for a demo.