4. Configuration
81
Communication
Time-out (ms)
Defines the application
level time-out
Maximum Number
of Retries
Defines the numbers of
retries before reporting a
communication error
Table 4-46. Device Configurations
Notes:
Slave Address: According to the MODBUS standard, the valid slave addresses are from 0 to 247,
where the addresses from 248 to 255 are reserved. When the master sends a writing command with
the address configured as zero, it is making broadcast requests in the network.
Communication Time-out: The communication time-out is the time that the master waits for a
response from the slave to the request. For a MODBUS RTU master device it must be taken into
account at least the following system variables: the time it takes the slave to transmit the frame
(according to the baud rate), the time the slave takes to process the request and the response sending
delay if configured in the slave. It is recommended that the time-out is equal to or greater than the
time to transmit the frame plus the delay of sending the response and twice the processing time of the
request. For more information, see Protocols Configuration-Communication Performance chapter.
Maximum number of retries: Sets the number of retries before reporting a communication error.
For example, if the slave does not respond to a request and the master is set to send three retries, the
error counter number is incremented by one unit when the execution of these three retries. After the
increase of the communication error trying to process restarts and if the number of retries is reached
again, new error will increment the counter..
Mappings Configuration – Symbolic Mapping Settings
The MODBUS relations configuration, showed on Figure 4-16, follows the parameters described on
Table 4-47:
Figure 4-16. MODBUS Data Mappings Screen
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