Modbus Double Register Format
January 09 UDA2182 Communications User Guide 19
Word Swap = No – Little Endian Format
The value 12345678 Hex will be represented as follows:
Bit 15 Bit 24
High
Low
High
Low
REGISTER N
(High)
REGISTER N+1
(Low)
Byte 2
56 Hex
Byte 1
78 Hex
Byte 4
12 Hex
Byte 3
34 Hex
Bit 8 Bit 23 Bit 16 Bit 31 Bit 0 Bit 7
7.4 Setting Up the Modbus Double Register Format
The UDA predominantly uses double registers for communicating data to software applications providing
Modbus/TCP protocol communications drivers. Double registers are sent as (2) consecutive 16-bit
registers, each register of which consists of two 8-bit bytes. Software packages require the registers and
bytes to be sent in a certain order. The UDA can be configured to deliver the data in two different word
orders. They are big endian and little endian.
Little Endian is the standard word order of the Modbus link.
Big Endian is the word order swapped. The UDA provides a configuration parameter to swap the word
order. This parmeter is called Word Swap.
Set it to no to use the Little Endian format.
Set it to yes to use the Big Endian format.
Selection Description Byte order Notes
No Little Endian 3, 4, 1, 2
Yes Big Endian 4, 3, 2, 1 UDA default
The word order is configurable using the front panel. The PIE tool cannot be used. To configure it using
the front panel, do the following:
1. On the front panel, select the Communication menu option in the Setup menu.
2. In the Communication menu, select Modbus.
3. Set Word swap to Yes for big endian, and No for little endian.
The Modbus TCP double register little endian transmission format (Word Swap = No), would be selected
for interfacing to most third party software packages. This is their standard.
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