When integrating a new Modbus slave with an unknown byte order, systematically testing the four combinations (ABCD, DCBA, BADC, CDAB) against a known float value is faster than relying on the incomplete or incorrect vendor documentation.

Vendor documentation is notoriously ambiguous on byte order: the phrase 'Big Endian' may mean ABCD or BADC depending on the author. The pragmatic field method is to know one value (e.g. a probed temperature of 22.5 degC), read the two registers, decode them with all four byte/word orders, and pick the combination that yields 22.5; once identified, that order is consistent across every float in the same slave. Useful tools include mbpoll with the -F flag, the pymodbus payload utilities, or a quick Python script around struct.unpack.

Mnemonic: ABCD = Big-Endian natural order, CDAB = word swap (most common in Modbus!), BADC = byte swap within each word, DCBA = full reversal (complete Little-Endian). Always carry a known reference value to a commissioning visit; one trusted reading lets you confirm the order in two minutes.