Endress+Hauser EngyVolt RV12, Руководство по эксплуатации

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EngyVolt RV12 Appendix
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This process is repeated until all eight shifts have been performed. After the last shift, the
next 8-bit message byte is exclusive OR'ed with the lower half of the 16 bit register, and
the process repeated. The final contents of the register, after all the bytes of the message
have been applied, is the error check value. In the following pseudo code “ErrorWord” is a
16-bit value representing the error check values.
BEGIN
ErrorWord = Hex (FFFF)
FOR Each byte in message
ErrorWord = ErrorWord XOR byte in message
FOR Each bit in byte
LSB = ErrorWord AND Hex (0001)
IF LSB = 1 THEN ErrorWord = ErrorWord – 1
ErrorWord = ErrorWord / 2
IF LSB = 1 THEN ErrorWord = ErrorWord XOR Hex (A001)
NEXT bit in byte
NEXT Byte in message
END
11.4.6 Function codes
The function code part of a Modbus Protocol message defines the action to be taken by the
slave. EngyVolt devices support the following function codes:
Code Modbus protocol name Description
03 Read Holding Registers Read the contents of read/write location (4X
references).
04 Read Input Registers Read the contents of read only location (3X
references).
08 Diagnostics Only sub-function zero is supported. This returns the
data element of the query unchanged.
16 Pre-set Multiple Registers Set the contents of read/write location (4X
references).
11.4.7 IEEE floating point format
The Modbus Protocol defines 16 bit “Registers” for the data variables. A 16-bit number
would prove too restrictive, for energy parameters for example, as the maximum range of
a 16-bit number is 65535. However, there are a number of approaches that have been
adopted to overcome this restriction. Integra Digital meters use two consecutive registers
to represent a floating-point number, effectively expanding the range to ±1 x 10
37
.
The values produced by EngyVolt devices can be used directly without any requirement to
“scale” the values, for example, the units for the voltage parameters are volts, the units for
the power parameters are watts etc.
What is a floating point number?
A floating-point number is a number with two parts, a mantissa and an exponent and is
written in the form 1.234 x 10 5 . The mantissa (1.234 in this example) must have the
decimal point moved to the right with the number of places determined by the exponent
(5 places in this example) i.e. 1.234 x 10 5 = 123 400.
If the exponent is negative the decimal point is moved to the left.
What is an IEEE 754 format floating-point number?
An IEEE 754 floating point number is the binary equivalent of the decimal floating-point
number shown above. The major difference being that the most significant bit of the