7SR120 Technical Manual
Chapter 4 - Page 30 of 84 © 2017 Siemens Protection Devices Limited
4.2 MODBUS Register Data Types
4.2.1 FLOAT_IEEE_754
The float data type conforms to the IEEE 754 floating point definition. This specifies that 32 bits of data will be
formatted as a sign bit in the most significant bit (MSB) followed by an 8 bit exponent then a 23 bit mantissa, down
to the least significant bit (LSB).
MSB LSB
Sign Exponent Mantissa
FLOAT_IEEE_754 IN DETAIL
The exponent is an 8 bit unsigned integer. To allow for negative exponents, it is offset by 127. Therefore
the actual exponent is e - 127. The following table shows a detailed layout of the exponent.
2
7
2
6
2
5
2
4
2
3
2
2
2
1
2
0
128 64 32 16 8 4 2 1
The mantissa contains the fractional part of a number normalized to the form 1.xyz i.e. in this instance
xyz. The mantissa represents the binary fraction of a number; therefore the MSB represents 2
-1
(or
1/2
1
) and its LSB 2
-23
(or 1/2
23
). The following table shows a detailed layout of the mantissa.
1
2
1
1
2
2
1
2
3
1
2
4
1
2
21
1
2
22
1
2
23
0.5 0.25 0.125 0.0625 4.768e-7 2.384e-7 1.192e-7
As an example 1,000,000 would be represented as follows (hex 49742400).
4 9 7 4 2 4 0 0
0 1 0 0 1 0 0 1 0 1 1 1 0 1 0 0 0 0 1 0 0 1 0 0 0 0 0 0 0 0 0 0
This calculates out as:
Sign = +1
Exponent = 10010010
2
= 128 + 16 + 2 = 146, subtract 127 = 19.
Mantissa = 1 +
1
2
1
+
1
2
2
+
1
2
3
+
1
2
5
+
1
2
10
+
1
2
13
= 1 +
4096 + 2048 + 1024 + 256 + 8 + 1
2
13
= 1 +
7433
2
13
= 1.907348632
Therefore Sign * 2
Exponent
* Mantissa = 1 * 2
19
* 1.907348632 = 1000000
FLOAT_IEEE_754 & MODBUS
In this MODBUS implementation the 32 bit float is stored in 2 16 registers in Big-Endian format. As an
example, if we take the hex representation of 1,000,000 as a float (from above) we have 49742400h.
Assume this is stored in the registers 30001 and 30002, it would look as follows.
Address Value
30001 4974
To Next Page
Loading...
Need help?
General
