Chapter 1. Introduction14 PACSystems* RX7i & RX3i TCP/IP Ethernet Communications User Manual GFK-2224Qcommunicating to a redundant system. Existing SRTP Server connections at the direct IP address are notdisturbed when the Ethernet interface switches between active and backup states.SRTP Client Operation in a Redundancy SystemOnly the active unit establishes and maintains SRTP Client connections (channels). The backup unit does notinitiate any SRTP Client operations. If SRTP Client operations are attempted, a COMMREQ error status is returnedto the local logic program. When the Ethernet interface changes from active to backup state, it takes down allSRTP Client connections and their underlying TCP connections.Because it can take some time to take down a TCP connection, the redundant system should reserve a spareSRTP Client connection for each connection using the Redundant IP address. That will prevent temporaryresource problems when establishing new SRTP Client connections to the new active unit while the previousconnections to the old active unit are being taken down.Modbus TCP Server Operation in a Redundancy SystemOnly the active unit maintains Modbus TCP Server connections at the Redundant IP address and is able torespond to Modbus TCP requests. The backup unit does not respond to the Redundant IP address. When anEthernet interface changes from active to backup state, it takes down all Modbus TCP Server connections andtheir underlying TCP connections that use the Redundant IP address.Remote hosts should use the Redundant IP address when communicating to a redundant system. ExistingModbus TCP Server connections at the direct IP address are not disturbed when the Ethernet interface switchesbetween active and backup states.Modbus TCP Client Operation in a Redundancy SystemOnly the active unit establishes and maintains Modbus TCP Client connections (channels). The backup unit doesnot initiate any Modbus TCP Client operations. If Modbus TCP Client operations are attempted, a COMMREQ errorstatus is returned to the local logic program. When the Ethernet interface changes from active to backup state,it takes down all Modbus TCP Client connections and their underlying TCP connections.Because it can take some time to take down a TCP connection, the redundant system should reserve a spareModbus TCP Client connection for each connection using the Redundant IP address. That will prevent temporaryresource problems when establishing new Modbus TCP Client connections to the new active unit while theprevious connections to the old active unit are being taken down.EGD Class 1 (Production & Consumption) in a Redundancy SystemThe active unit produces Ethernet Global Data exchanges to the network. The backup unit produces only theEGD exchanges for which Produce in Backup Mode is enabled. When the active Ethernet interfaces changes tobackup, it stops production of all EGD exchanges.When configured for Redundant IP operation, the active and backup Ethernet interfaces should be configured toconsume EGD exchanges via multicast host groups or the local subnet broadcast address. This permits both theactive and backup units to receive the latest data from the network. Unicast operation is not recommended. Thebackup unit does not consume any unicast exchanges at the Redundant IP address.EGD Class 2 Commands in a Redundancy SystemRemote hosts should use the Redundant IP address when communicating to a redundant system. Only theactive unit responds to EGD commands. The backup unit does not respond to the Redundant IP address. Whenthe active Ethernet interface changes to backup, any EGD command currently in process over the Redundant IPaddress is ended.When configured for Redundant IP operation, only the active unit sends EGD commands on the network. If thebackup unit tries to initiate any EGD commands, a COMMREQ error status is returned to its application program.When the active Ethernet interfaces changes to backup, any EGD commands in process are ended.Although not recommend, EGD commands may be issued to the direct IP address. Both the active and backupunits will respond to EGD commands received at the direct IP address.
