Siemens Argus 7SR21 Technical Manual page 416

7SR220 Technical Manual
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
Sign Exponent
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.
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
1/2 ) and its LSB 2
1 1
1 2
2 2
0.5 0.25 0.125
As an example 1,000,000 would be represented as follows (hex 49742400).
4 9
0 1 0 0 1 0 0 1
This calculates out as:
Sign = +1
Exponent = 10010010
Mantissa = 1 +
4096 + 2048 + 1024 + 256 + 8 + 1
= 1 +
Therefore Sign * 2
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.
Chapter 4 - Page 36 of 96
7 6 5 4
2 2 2 2
128 64 32 16
-23 23
(or 1/2 ). The following table shows a detailed layout of the mantissa.
1 1
3 4
2 2
0.0625
7 4
0 1 1 1 0 1 0
= 128 + 16 + 2 = 146, subtract 127 = 19.
2
1 1
+
1
2 2
13
2
Exponent
* Mantissa = 1 * 2
Address
30001
Mantissa
3 2 1
2 2 2 2
8 4 2 1
4.768e-7
2 4
0 0 0 1 0 0 1 0
1 1 1
+ + +
2 3 5 10
2 2 2
7433
= 1 + =
13
2
19
* 1.907348632 = 1000000
Value
4974
© 2017 Siemens Protection Devices Limited
0
1 1 1
21 22 23
2 2 2
2.384e-7 1.192e-7
0 0
0 0 0 0 0 0 0 0
1
+
13
2
1.907348632
LSB
-1
(or
0