Rockwell Automation T8xxx Trusted Communications Interface - Manual (Page 38)

Trusted
TM
Communication Interface T8151B
Issue 21 Apr 10 PD-T8151B 38
3.7. Modbus Master Redundant Configurations
Redundant systems can be implemented using multiple Communication Interfaces. Redundant links
can also be configured from a single Communications Interface, though this will result in the module
being a single point of failure. Facilities are provided to the application programmer to actively control
the use of the communications links, via application network variables defined by the user in the
system INI file, as described in section 3.5.
If a slave device fails to respond to three consecutive messages then a communications failure for that
slave will be logged (the number of messages may be configured in ‘Retries’). A communications error
to a slave device on a particular link results in the suspension of polling of that slave for a period of
time defined by the ping ‘Repeat Rate’. This allows the polling of other devices on the same master to
still be polled without pausing for the communications time-out on each cycle through the poll.
At regular intervals, the non-communicating slave is pinged by the master. Once the non-
communicating slave responds to a ping, then polling of the slave will recommence on the next poll
cycle. When a slave returns to the polling sequence, the first message that will be sent to the slave is
the first message defined in the INI file for that slave device.
Note: A slave replying with a Modbus exception message is not treated as having a communications
failure and will not be taken out of the polling cycle.