Dynasonics TFX Ultra Transit Time Flow Meters - Manual (Page 72)

COMMUNICATIONS PROTOCOLS
MODBUS
Available Data Formats
Bits Bytes Modbus Registers
Long Integer 32 4 2
Single Precision IEEE754 32 4 2
Double Precision IEEE754 64 8 4
Table 13: Available data formats
Modbus Register / Word Ordering
Each Modbus Holding Register represents a 16-bit integer value (2 bytes). The official Modbus standard defines Modbus as a
‘big-endian’ protocol where the most significant byte of a 16-bit value is sent before the least significant byte. For example,
the 16-bit hex value of ‘1234’ is transferred as ‘12’ ‘34’.
Beyond 16-bit values, the protocol itself does not specify how 32-bit (or larger) numbers that span over multiple registers
should be handled. It is very common to transfer 32-bit values as pairs of two consecutive 16-bit registers in little-endian word
order. For example, the 32-bit hex value of ‘12345678’ is transferred as ‘56’ ‘78’ ‘12’ ‘34’. Notice the Register Bytes are still sent in
big-endian order per the Modbus protocol, but the Registers are sent in little-endian order.
Other manufacturers, store and transfer the Modbus Registers in big-endian word order. For example, the 32-bit hex value of
‘12345678’ is transferred as ‘12’ ‘34’ ‘56’ ‘78’. It doesn’t matter which order the words are sent, as long as the receiving device
knows which way to expect it. Since it’s a common problem between devices regarding word order, many Modbus master
devices have a configuration setting for interpreting data (over multiple registers) as ‘little-endian’ or ‘big-endian’ word order.
This is also referred to as swapped or word swapped values and allows the master device to work with slave devices from
different manufacturers.
If, however, the endianness is not a configurable option within the Modbus master device, it’s important to make sure it
matches the slave endianess for proper data interpretation. The flow meter actually provides two Modbus Register maps to
accommodate both formats. This is useful in applications where the Modbus Master cannot be configured for endianness.
Communication Settings
Baud Rate 9600
Parity None
Data Bits 8
Stop Bits 1
Handshaking None
COMMUNICATIONS PROTOCOLS
Page 72 March 2014