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Modbus RTU control (RS-485 communication)
11.1.2 Communication timing
The communication time monitored by the driver and the communication timing of the master are as follows.
Query
P1
Tb2
C3.5
Tb4
C3.5C3.5
Tb3 (broadcast)
Query
Response
Slave
Code Name Description
Tb1
Communication
timeout
Intervals between received queries are monitored. If no query could be received after the time set with the
"RS-485 communication timeout alarm" parameter, an alarm of the communication timeout is generated.
[Initial value: 0 (Not monitored)]
When normal messages including messages to other slaves were received, the communication timeout
does not occur.
Tb2
Transmission
waiting time
This is the time after receiving a query from the master until it starts sending a response.
The transmission waiting time corresponds to the silent interval (C3.5) plus the value set in the
"Transmission waiting time" parameter (P1).
After the master sent a query, if the transmission waiting time is shorter than the time to switch to the
reception status, adjust with the "Transmission waiting time" parameter (P1). (Initial value: 3.0 ms)
Tb3
Broadcasting
interval
This is the time until the next query is sent in broadcasting.
A time equivalent to or longer than the silent interval (C3.5) plus 5 ms is required.
C3.5 Silent interval
This is the time after sending a response until switching to the reception status or after receiving a query
until switching to the transmission status.
*
It can be changed with the "Silent interval" parameter, but be sure to set the silent interval that is longer
than the time to send 3.5 characters. [Initial value: 0.0 (Set automatically)]
When the "Silent interval" parameter is set to "0.0: Set automatically," the silent interval varies depending
on the transmission rate. Refer to the following table for details.
Tb4
Transmission
waiting time
(master side)
This is the time after the master received a response until it send the next query (setting in the master side).
Set so that it is equal to or longer than the time set with the "Silent interval" parameter. Setting the "Silent
interval" parameter to "0.0: Set automatically" will be frame transmission intervals (reference) of the master
shown in the table below.
*
For the time after receiving a query until sending a response, the time for the "Transmission waiting time" parameter (P1) is
added.
When the "Silent interval" parameter is set to "0.0: Set automatically"
Transmission rate (bps) Silent interval Frame interval of the master (reference)
*
9,600 4.0 ms or more 5.0 ms or more
19,200
38,400
57,600
115,200
230,400
2.5 ms or more 3.0 ms or more
*
The driver can receive a query certainly by setting the frame transmission intervals of the master (the time after receiving a
response until sending) to the reference time.
Note
•
After the master sent a query, if the transmission waiting time is shorter than the time to switch to the reception
status, a response may not be received or the data may be missed.
•
When the timeout period is set in the mater, set it to be equal to or longer than the time corresponding to the silent
interval (C3.5) plus the value set in the "Transmission waiting time" parameter (P1).
•
If frames are received at an interval shorter than the required silent interval, the frames are discarded and a
communication error occurs. When a communication error occurs, check the silent interval of the slave and reset
the frame transmission interval.
•
The silent interval may vary depending on the product series connected. When connecting multiple product series,
set parameters as follows.
- "Silent interval" parameter: "0.0: Set automatically"
- "Transmission waiting time" parameter: 1.0 ms or more
•
If the setting of the "Silent interval" parameter is common to systems in which only products with the "Silent
interval" parameter are connected, the communication cycle can be improved. Use in a state of setting to "0.0: Set
automatically" normally.
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