Byte and Word swap when dealing with 32 bits (4 Bytes) Modbus data

Note 1 – Article used as reference

http://www.chipkin.com/how-real-floating-point-and-32-bit-data-is-encoded-in-modbus-rtu-messages/

Note 2 - Software used as reference

FPconverter from http://www.61131.com

Modbus protocol was designed based on devices with a 16-bit register length. Consequently, special considerations were required when implementing 32-bit data elements. This implementation settled on using two consecutive 16-bit registers to represent 32 bits of data or essentially 4 bytes of data.

Big-Endian is the most commonly used format for network protocols – so common, in fact, that it is also referred to as ‘network order’.

As a rule of thumb, the family of a device’s microprocessor determines its endianness. Typically, the big-Endian style (the high-order byte is stored first, followed by the low-order byte) is generally found in CPUs designed with a Motorola processor. The little-Endian style (the low-order byte is stored first, followed by the high-order byte) is generally found in CPUs using the Intel architecture.

System integrator has to be very careful when dealing with 32 bits Modbus data. Various manufacturers may implement various data swapping techniques as described in table below:

Swap Mode	Source Bytes		Target Bytes
None	AB	CD	AB	CD
Byte and Word swap	AB	CD	DC	BA
Byte swap	AB	CD	BA	DC
Word swap	AB	CD	CD	AB