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2920 Float & Tape Transmitter MODBUSAppendix A — MODBUS Implementation
154 Installation and Operations Manual
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 2920
FTT that are not easily scaled (such as the scaling factors themselves).
Floating-point Data
The 2920 FTT 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 mea-
surement unit selected in the 2920 FTT be the same as that expected by the host. In addi-
tion, where possible, data is available in both integer and floating-point formats.
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 2920 FTT is shown as follows:
Example of Floating-point Data
• Hexadecimal representation: 42 C8 00 00
• Decimal equivalent: 100.00
Floating-point registers that are defined as reserved will respond with undocumented values.
A write command to a reserved floating-point register is ignored.
The one 32-bit register format
Function code 65 (41 Hex) is used to read floating-point registers in this format. An example of
a floating-point register read is shown below with the a temperature of 100.0 degrees is being
read from register 1 (0001 Hex).
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 24 14
2920 FTT Response
Address Function
Code Byte Count Data
MSB
Data
LSB Data
MSB Data
LSB Error
Check
01 03 04 42 C8 00 00 6F B5