D-Link DE-805TP - 10Mbps Ethernet Mini Hub User Manual download pdf (Page 69)

Modbus for Field Technicians

Page 69

21. HOW REAL (FLOATING POINT) AND 32-BIT DATA IS

ENCODED IN MODBUS RTU MESSAGES

The article discusses some of the typical difficulties encountered when

handling 32-bit data types via Modbus RTU and offers practical help for

solving these problems.

The point-to-point Modbus protocol is a popular choice for RTU

communications if for no other reason that it’s basic convenience. The

protocol itself controls the interactions of each device on a Modbus network,

how device establishes a known address, how each device recognizes its

messages and how basic information is extracted from the data. In essence,

the protocol is the foundation of the entire Modbus network.

Such convenience does not come without some complications however, and

Modbus RTU Message protocol is no exception. The protocol itself was

designed based on devices with a 16-bit register length. Consequently, special

considerations were required when implementing 32-bit data elements. This

implementation settled on using two consecutive 16-bit registers to represent

32 bits of data or essentially 4 bytes of data. It is within these 4 bytes of data

that single-precision floating point data can be encoded into a Modbus RTU

message.

THE IMPORTANCE OF BYTE ORDER

Modbus itself does not define a floating point data type but it is widely

accepted that it implements 32-bit floating point data using the IEEE-754

standard. However, the IEEE standard has no clear cut definition of byte order

of the data payload. Therefore the most important consideration when

dealing with 32-bit data is that data is addressed in the proper order.

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D-Link DE-805TP - 10Mbps Ethernet Mini Hub User Manual Page 69