Chapter 1. IntroductionGFK-2224Q January 2017 13Role Switching in Non-HSB Redundancy SystemsWhen redundant IP is enabled for an Ethernet module in a non-HSB CPU system, it is the responsibility ofapplication logic to set the redundancy mode of the Ethernet module. The Set Application Redundancy ModeService Request (SVC_REQ 55) instruction is used to inform the Ethernet module of the current redundancy roleof the host CPU. This SVC_REQ should be used to provide redundancy role switch notification to all Ethernetinterfaces in the controller that are configured for redundant IP operation.After commanding a role switch for an Ethernet interface, the application logic can monitor the module’s LANInterface Status (LIS) block to determine when it has activated the Redundancy IP address. For details about theLIS, refer to “Monitoring the Ethernet Interface Status Bits” in Chapter 12Note: The application must allow sufficient time for Redundant IP activation (at least 120msec) beforecommanding another redundancy role switch.When an Ethernet interface recognizes that a redundant IP address has been configured for it, the modulesends a mail message to the CPU to register for redundancy role switch notification. In non-HSB systems, theEthernet interface is initially put into backup mode. After power up, the application logic must use a SVC_REQ toset the redundancy state to the desired value. Once running, the CPU remembers the last commandedredundancy role sent to that Ethernet interface. When an Ethernet interface is restarted, the CPU automaticallycommands the Ethernet interface to its last redundancy state without explicit action by the application logic.Going to Stop ModeWhen a non-HSB CPU goes to Stop mode, Ethernet interfaces that are configured for redundant IP areautomatically set to backup mode. When the CPU is subsequently returned to Run mode, the Ethernet interfacesremain in backup mode until the application logic sets the redundancy mode to active.Stop/IO Scan Enabled ModeIn this mode, I/O scanning including EGD service continues when the non-HSB CPU is stopped. However,Ethernet interfaces configured for redundant IP operation are automatically set to backup mode and normalEGD production for those interfaces is stopped. Only the EGD exchanges with Produce in backup mode enabledare produced while the CPU is in Stop/IO Scan Enabled mode. To stop production for all EGD producedexchanges including Produce in backup mode exchanges, choose the Stop/IO Scan Disabled mode of operation.Commanding a Role Switch in a Non-HSB Redundancy SystemUse the Set Application Redundancy Mode service request (SVC_REQ 55) with non-HSB CPUs to request that theCPU send redundancy role switch commands to all Ethernet interfaces in that PLC that are configured forredundant IP operation. For details on using the Service Request function, refer to the PACSystems CPUReference Manual, GFK-2222.This function has an input parameter block with a length of one word.address 0=Backup redundancy role1=Active redundancy roleSVC_REQ 55 is recognized in non-HSB CPUs only. This service request sends a role switch command to allEthernet interfaces in the PLC that are configured for redundant IP operation. The application must monitor theLAN Interface Status (LIS) word for each Ethernet interface to determine whether the Redundant IP address isactive at that interface.SVC_REQ 55 has no effect on Ethernet interfaces that are not configured for redundant IP operation.SRTP Server Operation in a Redundancy SystemOnly the active unit maintains SRTP Server connections at the Redundant IP address and is able to respond toSRTP requests. The backup unit does not respond to the Redundant IP address. When an Ethernet interfacechanges from active to backup state, it takes down all SRTP Server connections and their underlying TCPconnections that use the Redundant IP address.Both the active and backup units maintain SRTP Server connections at the direct IP address for networkcommunication with the programmer. Other remote hosts should use the Redundant IP address when
