Appendix
EngyVolt RV15
52
S = represents the sign bit where 1 is negative and 0 is positive
E = is the 8-bit exponent with an offset of 127 i.e. an exponent of zero is represented by 127, an
exponent of 1 by 128 etc.
M = the 23-bit normal mantissa. The 24th bit is always 1 and, therefore, is not stored.
Using the above format the floating point number 240.5 is represented as 43708000 hex:
Data Hi Reg, Hi Byte.
Data Hi Reg, Lo Byte.
Data Lo Reg, Hi Byte.
Data Lo Reg, Lo Byte.
43
70
80
00
The following example demonstrates how to convert IEEE 754 floating-point numbers from their
hexadecimal form to decimal form. For this example, we will use the value for 240.5240.5 shown
above.
The floating-point storage representation is not an intuitive format. To convert this value to
decimal, the bits should be separated as specified in the floating-point number storage format
table shown above.
For example:
Data Hi Reg, Hi Byte.
Data Hi Reg, Lo Byte.
Data Lo Reg, Hi Byte.
Data Lo Reg, Lo Byte.
0100
®
0011
0111
®
0000
1000
®
0000
0000
®
0000
From this you can determine the following information.
• The sign bit is 0, indicating a positive number.
• • The exponent value is 10000110 binary or 134 decimal. Subtracting 127 from 134 leaves 7,
which is the actual exponent.
• • The mantissa appears as the binary number 11100001000000000000000
There is an implied binary point at the left of the mantissa that is always preceded by a 1. This bit
is not stored in the hexadecimal representation of the floating-point number. Adding 1 and the binary
point to the beginning of the mantissa gives the following: 1.11100001000000000000000
Now, we adjust the mantissa for the exponent. A negative exponent moves the binary point to the
left. A positive exponent moves the binary point to the right. Because the exponent is 7, the mantissa
is adjusted as follows: 11110000.1000000000000000.
Finally, we have a binary floating-point number.
Binary bits that are to the left of the binary point represent the power of two corresponding to their
position. The result is the following decimal value:
.11110000 = (1 x 2
7
) + (1 x 2
6
) + (1 x 2
5
) + (1 x 2
4
) + (0 x 2
3
)+ (0 x 2
2
) + (0 x 2
1
)+ (0 x 2
0
) =
240
Binary bits that are to the right of the binary point also represent a power of 2 corresponding to their
position. As the digits are to the right of the binary point the powers are negative. For example: .
100… = (1 x 2
-1
) + (0 x 2
-2
)+ (0 x 2
-3
) + … = 0.5
Adding these two numbers together and making reference to the sign bit produces the number
+240.5.
For each floating point value requested two Modbus Protocol registers (four bytes) must be requested.
The received order and significance of these four bytes for EbgyVolt devices is shown below:
Data Hi Reg, Hi Byte.
Data Hi Reg, Lo Byte.
Data Lo Reg, Hi Byte.
Data Lo Reg, Lo Byte.
11.4.8 Modbus commands supported
All EngyVolt devices support the “Read Input Register” (3X registers), the “Read Holding Register”
(4X registers) and the “Pre-set Multiple Registers” (write 4X registers) commands of the MODBUS
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