Yokogawa PR300 Power and Energy Meter - Manual (Page 83)

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<Toc> <Ind> <5. Modbus/RTU and ASCII Communication Protocols>
IM 77C01E01-10E
5.1.2 Specifying D Registers
Specification of D registers using commercially available SCADA or other software
and specification of D registers for messages used in a customer-created communi-
cation program are different. Take note of this.
For a customer-created communication program, specify a value in hexadecimal that is
obtained by subtracting 40001 from a reference number.
Example: Specifying “D0043”
For messages in the customer-created communication program, specify “002A,” the
hexadecimal number of value 42 obtained by subtracting 40001 from the reference num-
ber.
5.1.3 Checking Errors
Modbus communication has two modes, i.e., the ASCII mode which is communication
based on ASCII characters and the RTU mode which is binary code communication. These
modes use different error-checking methods.
■
ASCII Mode
In the ASCII mode, errors are checked by means of an LRC longitudinal redundancy
check.
The LRC value is the two's complement of the sum obtained by adding up data byte by
byte, from the station number to the last data item, excluding ‘:’, ‘CR’ and ‘LF.’ Ignore the
carry that may occur at the upper digit when adding up the data.
Example:
The method of calculating the LRC for the [:]110300C80004[LRC][CR][LF] command for
reading a series of four D registers of the device at station number 17, starting with the
D00201 (VT ratio) register, is as follows:
[1] Station number 17 is 11 in hexadecimal.
Change the data to byte-by-byte hex data
⇒
11,03,00,C8,00,04.
(In the Modbus/ASCII message, this data is represented by the ASCII code as two
bytes, i.e., 31 and 31 in hexadecimal.)
[2] Add up the byte-by-byte hex data on a byte-by-byte basis.
⇒
11 + 03 + 00 + C8 + 00 + 04 = E0
[3] Find the two's complement of the lower one byte of the data thus added up ⇒ 20
11100000 (0xE0)
→
00011111 (complement) + 1 = 00100000 (20)