Opto 22 posted Configuring Modbus Protocol 32-bit Word Order Opto 22 Woodland, WA:

Configuring Modbus Protocol 32-bit Word Order

Background on Modbus Protocol and Big-Endian vs. Little-Endian Word Order
The 1970's Modbus protocol standards give guidance on the protocol, but they don't address the word order beyond the 16-bit register level that was available at that time. For the 16-bit register values, the most significant byte always precedes the least significant byte (this is referred to as Big-Endian word order).
When 32-bit (4 byte) values came about later, the least significant word was mapped onto the lower address of the register pair and was referred to Little-Endian word order). This lack of standardization for values larger than 16 bits…

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