Echelon SmartServer 2.2 - Manual (Page 282)

SmartServer 2.2 User’s Guide 283
Data Ordering
Scheme Select the ordering scheme to be used for interpreting Modbus data. You
have the following four choices:

Big Endian. The highest order byte of data is sent first and all
subsequent bytes of data are arranged from highest to lowest order.
This is the default.

Little Endian. Lowest order byte of data is sent first, and each
subsequent byte is arranged from lowest to highest order

Byte Swapped. Data is first arranged from highest to lowest order,
but every pair of bytes in the structure is interchanged.

Word Swapped. Data is first arranged from highest to lowest order,
but every pair of 16-bit words is swapped.
For example, consider a device that uses an unsigned 32-bit integer to
report runtime accumulation. Selecting the data ordering scheme is
required because the Modbus protocol leaves the interpretation of 32-bit
integers to the discretion of the implementer.
In Big Endian format, the value of 120,000 hours (0x01D4C0 in
hexadecimal format) would be represented as a value of: 00 01 D4 C0 in
memory. This requires two adjacent Modbus registers (each holding 16
bits of data). If the device manufacture defines the unit runtime to be at
register address 0x8, the Big Endian formatted response to a read function
would return data 0x0001 0xD4C0 in that order.
Now suppose the manufacture states the U32 value is returned in Little
Endian format. One interpretation of the value returned to the driver from
the read function would be 0xC0D4 0x0100.
Alternatively, the manufacture may interpret Little Endian to be the
ordering of registers and not bytes. In this case, the read function would
return 0xD4C0 0x0001 and the driver would need to swap words to handle
the value. If the device returned a value of 0x0100 0xC0D4A, a byte
swapped format would need to be applied.
Viewing Modbus Data Points
You can view and configure all the data points on a Modbus device using the Overview - Data Points
Web page. This Web page includes sortable columns for the addressing properties, register type, poll
rate, and the format and type parameters of each data point on a Modbus device. This Web page is
especially useful for setting the start addresses of the data points on a Modbus device.
To use the Overview - Data Points Web page to configure the data points on a Modbus device, follow
these steps:
1. To configure the addressing properties, register types, names, poll rates, or descriptions of the
Modbus data points, click the Driver option at the top of the navigation pane on the left side of the
SmartServer Web interface.
2. Right-click a Modbus device, point to Overview, and then click Data Points on the shortcut
menu.