Instruction manual

4000 Modbus Implementation

9.6 Floating-Point Registers

Although not part of the Modbus protocol specification, floating point numbers have

been implemented using the IEEE 754 standard. Floating point numbers reduce the

complexity required in scaling integer values and provide a means to transmit numbers

used by the ATT that are not easily scaled (such as the scaling factors themselves).

9.6.1 Floating-Point Data

The ATT is capable of using a two 16-bit registers format and a one 32-bit register

format. Examples and descriptions of both formats follow.

Note! Although this type of data does not require scaling, it is important that the measure-

ment unit selected in the ATT be the same as that expected by the host. In addition,

where possible, data is available in both integer and floating-point formats.

9.6.2 The Two 16-bit Registers Format

Function code 03 or 04 is used to read floating-point registers in this format. Function

code 16 is used to write floating-point registers in this format. An example of reading

the temperature, register 102 ( 0066 Hex), in floating-point format from the ATT is

shown as follows:

9.6.3 Floating Point Data

• Hexadecimal representation: 42 C8 00 00

• Decimal equivalent: 100.00

Floating-point registers that are defined as reserved have zero (0) as their only legal

value. A write command to a reserved floating-point register is ignored.

Host Request

Address Function

Code

Start Reg

# of Reg

Error

Check

01 03 00 66 00 02 XX

ATT Response

Address Function

code

Byte

Count

Data

MSB

Data

LSB

Data

MSB

Data

LSB

Error

Check

01 03 04 42 C8 00 00 XX