It is also possible to have a HA Modbus system without connecting any field devices, CI521-MODTCP / CI522-MODTCP in the network. This system can be used for establishing a redundant PLC system with data synchronization between two AC500 controllers, either without field IO or with user integration of other protocols to field-IO or “intelligent” IO: CPUs as field devices.

Secondary will be on hot standby with primary PLC, during a power off/ stop of the primary PLC. Secondary will take over the control and continue the process. Any user integrated field-IO or CPUs can establish communication mapped with the primary bit: parallel reading but prevent parallel writing.

HA without CI modules can be also used during commissioning to check the data sync, OPC and SCADA related communications without any field devices configured. The user has to set the global variable 'xNoCiBus' to TRUE defined in the HA_GLOBAL_VARIABLES. This variable has to be set to TRUE in both PLCs. Note: It is not advised to update this variable during run time.