Endress+Hauser EngyVolt RV12, Operating Instructions Manual

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EngyVolt RV12 Appendix
51
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
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 Protocol RTU protocol. All values stored and returned are in
floating point format to IEEE 754 with the most significant register first.
Read input registers
MODBUS Protocol code 04 reads the contents of the 3X registers.
Example: The following query will request ‘Volts 1’ from an instrument with node address
1:
Field name Example (Hex)
Slave address 01
Function 04
Starting address High 00
Starting address Low 00
Number of points High 00
Number of points Low 02
Error check Low 71
Error check High CB
Data must be requested in register pairs i.e. the “Starting Address“ and the “Number
of Points” must be even numbers to request a floating point variable. If the “Starting
Address” or the “Number of points” is odd then the query will fall in the middle of a
floating point variable the product will return an error message.
The following response returns the contents of Volts 1 as 230.2.
But see also “Exception Response” later.
Field name Example (Hex)
Slave address 01
Function 04
Byte count 04
Data, High Reg, High Byte 43
Data, High Reg, Low Byte 66
Data, Low Reg, High Byte 33
Data, Low Reg, Low Byte 34