Varec Network Adapter 4000, Installation And Operation Manual, Page: 63 / 80

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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
H Start Reg
L # of Reg
H # of Reg
L 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