KW1M-H Eco-POWER METER
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7.4 MODBUS (RTU) Communication
7.4.1 Overview of MODBUS (RTU)
◆
8-bit binary data in command is transmitted as it is.
Data format
Start bit
: 1 bit
Data bit
: 8 bits *7bits is not available.
Parity
: No parity, Even parity, Odd parity Selectable
Stop bit
: 1 bit (Fixed)
Error detection
: CRC-16 (Cyclic Redundancy Check)
Data interval
: 3.5 character transmission time or less
◆
Message configuration
RTU mode is configured to start after idle time processing of more than 3.5 character transmissions
and end after idle time processing of more than 3.5 character transmissions.
3.5 idle
characters
Slave
address
Function
code
Data
Error check
CRC-16
3.5 idle
characters
8-bit
8-bit
＊＊
bits
16-bit
Master judges the transmission complete after no command for 4-characters idle time and process the
command.
*Transmission speed and judgment time to complete transmission
Transmission speed (bps)
Judgment time to complete (ms)
38400
about 1.00
19200
about 2.00
9600
about 4.00
4800
about 8.00
2400
about 16.00
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Slave address:
Slave address is an individual instrument number on the slave side and is set within the range 1 to 99
(01H to 63H). Master identifies slaves by the slave address of the requested message.
Slave informs master which slave is responding to master by placing its own address in the response
message. Slave address 0 (00H, broadcast address) can identify all slaves connected. However
slaves do not respond.
◇
Function code: Function code is command code for the slave to undertake the following action types.
Function code
Contents
03(03H)
DT Read
06(06H)
DT 1 word write
16(10H)
DT several data write
Function code is used to discern whether the response is normal (acknowledgement) or if any error
(negative acknowledgement) has occurred when slave returns response message to master.
When acknowledgement is returned, slave simply returns original function code. When negative
acknowledgement is returned, MSB of original function code is set as 1 for response.
For example, when the master sends request message setting 00H to function code by mistake, slave
returns 80H by setting MSB to 1, because the former is an illegal function.
For negative acknowledgement, the exception codes below are set to data of response message and
returned to master in order to inform it of what kind of error has occurred.
Exception code
Contents
1(01H)
Illegal Function (Non-existent function)
3(03H)
Illegal data value (Value out of the devise numbers)
note1) Even if it commands to write (06H.10H) to non-existent data address, slave response with
acknowledgement. However, it doesn’t write.
note2) Even if it commands to write the value out of the setting range, slave response with
acknowledgement. However, it doesn’t write.
note3) The maximum number of reading slaves is 26 (57 bytes), the maximum number of writing
slaves is 23 (55 bytes).
