Non-Hsb Redundancy; Effect Of Redundancy Role Switching On Ethernet Communications; Figure 3: Basic Non-Hsb System With Redundant Ip - GE RX3i User Manual

Chapter 1. Introduction

Non-HSB Redundancy

Non-HSB redundancy systems use RX7i or RX3i CPUs that do not have specialized firmware for controlling
redundancy operations. (These CPUs have a "CPE" or "CPU" designation.) In these systems, the application logic
coordinates between CPUs that act as redundant partners, and determines which CPU is the active unit and
which are backup units. The figure below illustrates the use of the redundant IP feature in a non-HSB
redundancy system. Two non-HSB CPUs (designated primary and secondary) are linked by a communications
connection. An Ethernet interface in each controller is configured with Redundant IP enabled so that they share
a Redundant IP address. As in an HSB system, only the active Ethernet interface can communicate through the
Redundant IP address to produce EGD exchanges or to initiate Channel operations.
The application logic must monitor the status of the Ethernet modules in the system to manage the
active/backup status of each controller.
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Effect of Redundancy Role Switching on Ethernet Communications

When a redundancy role-switch occurs, Ethernet communications switch to the backup unit, which has no
knowledge of any communication state at the previously-active unit. The application must include logic to
detect loss of communication during a redundancy role switch and to then reinitiate communication.
To remote hosts on the network, the redundant system is viewed as a single PLC with high reliability; the remote
host neither knows nor cares which PLC is the active unit. By using the Redundant IP address, the remote host
always communicates with the active unit. When a redundancy role switch occurs, the formerly-active PLC
gives up ownership of the Redundant IP address and takes down all connection-oriented communications
currently using the Redundant IP address. The applications in the redundant system and remote hosts must
reestablish any such communications; the new Redundant IP connections will use the newly active PLC.
The programmer can still communicate directly with each PLC in the redundant system (for example, to store
new logic or configuration) using the direct IP address of each Ethernet Interface.
Role Switching In HSB Redundancy Systems
In HSB redundancy systems, a role switch is initiated automatically by the redundancy CPU when the active unit
detects a fatal fault, is placed in Stop mode, or is powered off. An HSB role switch can also be initiated manually
or by the application logic. For additional information about role switches in HSB systems, refer to the
PACSystems Hot Standby CPU Redundancy User's Guide, GFK-2308.
12 PACSystems* RX7i & RX3i TCP/IP Ethernet Communications User Manual
Primary Controller
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Remote Device

Figure 3: Basic non-HSB System with Redundant IP

Secondary Controller
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Ethernet
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