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• Details of protocol
The following table explains the four message fields.
Start
T1
Message field
Address field
Function field
Data field
CRC check field
Function code list
Function name
Read holding register
Preset single register
Diagnostics
Preset multiple
registers
Read holding register
access log
Read holding register (reading data of holding registers) (H03 or 03)
• Query message
a. Slave address
(8 bits)
134
5. PARAMETERS
5.7 (N) Communication operation parameters
Address
Function
8 bits
8 bits
"0 to 247" can be set in the single-byte (8-bit) length field. Set "0" when sending broadcast messages
(instructions to all addresses), and "1 to 247" to send messages to individual slaves.
The response from the slave also contains the address set by the master. The value set in Pr.331 RS-485
communication station number is the slave address.
"1 to 255" can be set as the function code in the single-byte (8-bit) length filed. The master sets the function to
be sent to the slave as the request, and the slave performs the requested operation. Refer to the function code
list for details of the supported function codes. An error response is generated when a function code other than
those in the function code list is set.
The normal response from the slave contains the function code set by the master. The error response contains
H80 and the function code.
The format changes according the function code. (Refer to
count, number of bytes, and accessing content of holding registers.
Errors in the received message frame are detected. Errors are detected in the CRC check, and the 2 bytes length
data is appended to the message. When the CRC is appended to the message, the lower bytes of the CRC are
appended first, followed by the upper bytes.
The CRC value is calculated by the sender that appends the CRC to the message. The receiver recalculates
the CRC while the message is being received, and compares the calculation result against the actual value that
was received in the error check field. If the two values do not match, the result is treated as an error.
Read/
Code
write
The data of the holding registers is read.
The various data of the converter unit can be read from MODBUS
registers.
System environmental variable (Refer to
Read
H03
Real time monitor (Refer to
Fault history (Refer to
Model information monitor (Refer to
Converter unit parameters (Refer to
Data is written to a holding register.
Data can be written to MODBUS registers to output instructions
Write
H06
to the converter unit or set parameters.
System environmental variable (Refer to
Converter unit parameters (Refer to
Functions are diagnosed. (Communication check only)
A communication check can be made since the query message
Read
H08
is sent and the query message is returned as it is as the return
message (subfunction code H00 function).
Subfunction code H00 (Return query data)
Data is written to multiple consecutive holding registers.
Data can be written to consecutive multiple MODBUS registers to
Write
H10
set converter unit parameters.
System environmental variable (Refer to
Converter unit parameters (Refer to
The number of registers that were successfully accessed by the
previous communication is read.
Queries by function codes H03 and H10 are supported.
Read
H46
The number and start address of holding registers successfully
accessed by the previous communication are returned.
"0" is returned for both the number and start address for queries
other than function code H03 and H10.
b. Function
c. Starting address
H03
H
(8 bits)
(8 bits)
Data
L
n × 8 bits
8 bits
Description
page
Outline
page
139.)
page
99.)
page
140.)
page
140.)
page
140.)
page
139.)
page
140.)
page
139.)
page
140.)
d. No. of points
L
H
(8 bits)
(8 bits)
CRC check
H
T1
8 bits
134.) The data, for example, includes the byte
Broadcast
communication
Not available
Available
Not available
Available
Not available
CRC check
L
L
(8 bits)
(8 bits)
End
Message
format
reference
page
page 134
page 135
page 136
page 136
page 137
H
(8 bits)
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