Rockwell Automation AADvance Controller Configuration Guide - Manual (Page 193)

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Document: 553633
ICSTT-RM405f-EN-P
Issue 07
9-7
8 Slave 2 Message 1 Repeat message
9 Slave 3 Message 3 End of pass through slave 3
10 Slave 1 Message 4 End of pass through slave 1
11 Slave 2 Message 2 Repeat message
12 Slave 3 Message 1 Repeat message; end of poll
Handling Modbus Communication Errors
If a Modbus master encounters communication errors to a slave device, it suspends
polling of that slave for a period of time. This allows the polling of other slaves on the
same master to continue without pausing for the communications time-out on each
cycle through the poll. At regular intervals, the controller can ping the non-
communicating slave (this is a configurable setting); once the non-communicating slave
responds to a ping, then polling of the slave will recommence on the next cycle.
Note: If the controller is receiving error messages in the replies from a slave, the slave
remains in the polling cycle; the controller does not deem this to be a communication
failure.
Modbus Statistics
The AADvance controller maintains a cycle timer and counters to generate statistics
for each slave. The statistics are available to the application, so the application can
react accordingly or report how well a link is operating; the statistics are also used by
diagnostic commands. The application can read the statistics as variables (you specify
the address of each variable) and can also reset the statistics to zero. Statistics are
available for all point-to-point message transfers, but not for broadcast messages.
Three kinds of statistics are available:
 Last rate, which reports the length of the most recent scan time
 Maximum rate, which reports the longest scan time
 Average rate, which reports the average scan time
The statistics are reported in hundredths of a second (not milliseconds).
In operation, the statistics are updated on the first message of each slave and on the
first slave link of each master to produce the slave and master cycle times. The
software also checks the the application interface to see if a statistics reset is required.
Meanwhile, the Modbus master measures the overall scan time; this is from the first
slave start, to round to first slave start again. Again, the statistics are updated on the
first message of each slave and the first slave of each master to produce the slave and
master cycle times, and the application interface is checked to see if a statistics reset is
required.