Modbus (Rtu) Communication; Overview Of Modbus (Rtu) - Panasonic kw9m User Manual

6.4 MODBUS (RTU) Communication

6.4.1 Overview of MODBUS (RTU)

◆8-bit binary data in command is transmitted as it is.
Data format
◆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
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)
38,400
19,200
9,600
4,800
2,400
1,200
◇Slave address:
Slave address is an individual instrument number on the slave side and is set within the range 1 to
247 (01H to F7H) for Modbus communication. 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
03(03H)
06(06H)
16(10H)
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
1(01H)
3(03H)
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-byte), the maximum number of writing slaves
is 23 (55-byte).
Start bit
Data bit
Parity
Stop bit
Error detection
Data interval
Slave Function
address code
8-bit 8-bit
Judgment time to complete (ms)
DT Read
DT1 word write
DT several data write
Illegal Function (Non-existent function)
Illegal data value (Value out of the devise numbers)
: 1 bit
: 8 bits (Fixed)
: No parity, Even parity, Odd parity Selectable
: 1-bit, 2-bit Selectable
: CRC-16 (Cyclic Redundancy Check)
: 3.5 character transmission time or less
Error check
Data
CRC-16
xx bits 16-bit
about 1
about 2
about 4
about 8
about 16
About 32
Contents
Contents
48
KW9M Eco-POWER METER
3.5 idle
characters