D-Link DE-805TP - 10Mbps Ethernet Mini Hub User Manual download pdf (Page 71)

Modbus for Field Technicians

Page 71

Furthermore, both a “byte swap” and a “word swap” would essentially

reverse the sequence of the bytes altogether to produce yet another result:

Clearly, when using network protocols such as Modbus, strict attention must

be paid to how bytes of memory are ordered when they are transmitted, also

known as the ‘byte order’.

DETERMINING BYTE ORDER

The Modbus protocol itself is declared as a ‘big-Endian’ protocol, as per the

Modbus Application Protocol Specification, V1.1.b:

“Modbus uses a “big-Endian” representation for addresses and data items.

This means that when a numerical quantity larger than a single byte is

transmitted, the most significant byte is sent first.”

Big-Endian is the most commonly used format for network protocols – so

common, in fact, that it is also referred to as ‘network order’.

Given that the Modbus RTU message protocol is big-Endian, in order to

successfully exchange a 32-bit datatype via a Modbus RTU message, the

endianness of both the master and the slave must be considered. Many RTU

master and slave devices allow specific selection of byte order particularly in

the case of software-simulated units. One must merely insure that both all

units are set to the same byte order.

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D-Link DE-805TP - 10Mbps Ethernet Mini Hub User Manual Page 71