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Appendix C
February 2019
Figure 9-8: Dual RS-485 Architecture
A. Engineering station
B. Process control network
C. Controller and I/O
D. Dual serial card
E. RS-485 bus
F. Secondary Ethernet
Note
By default, only the active Gateway in a redundant system will respond to Modbus polling
requests. If simultaneous polling is desired, login to the Gateway web interface, navigate
to Setup>Modbus>Communications and set "Respond when running as redundant
standby?" to Yes. Only use this setting in a dual RS-485 architecture.
Power
Power for the redundant Gateways should be applied after all primary and secondary
Ethernet and RS-485 connections have been made. Using separate uninterruptable power
supplies (UPS) is recommended to ensure availability of the redundant Gateway system.
9.5
Diagnostics
The redundant system will perform many diagnostic checks to verify the health and
connectivity of the system. In the event of a failure, it can take up to 30 seconds for the
Gateway to trade positions.
These diagnostics can also be mapped to Modbus registers or OPC tags. The following
table covers what diagnostics are included on the Redundancy Status page as well as how
they can be mapped as parameters in Modbus or OPC.
62
Reference Manual
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