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6.1.4
Modbus RTU Communication Interval
This section describes the Modbus RTU (RS-485) communication interval
.
Current request
Data
Start
Idling
time(>3.5
letters)
Unit
ID
Function
code
Data
CRC(L)
CRC(H)
End
Idling
time(>3.5
letters)
Data
▲ Previous response message ▲Error check code
▲Current response message
Provide a non-communication interval (idling time of more than 3.5-letter) between message
frames, to identify the end and start of each message.
The slave returns a response message after non-communication interval (idling time of more than
3.5-letter) from its receipt of a request message.
If a space of more than 3.5 letters is detected within a message frame, the slave judges that the
message frame ends then discards the message, then judges the next received data to be the
start of the request message.
Idling time calculation method
For example, when the communication speed is 9600bps and 1 letter is 11-bit long
(Modbus RTU communication data),
Interval per letter = 1 /9600 x 11 x 1000
≈
1.15 msec.
If converted to 3.5 letters, 1.15 x 3.5
≈
4.01 msec.
NOTE
