1. INTRODUCTION
1.1.4.9.
CPU Redundancy
For very critical applications, Nexto Series is capable of having redundancy of CPUs. The only CPU model with this
feature is NX3030. These CPUs can be located in different racks (known as half clusters). In this architecture, the system
will have one controller running the control task (primary controller) and another one acting as the standby controller with all
current system status for automatic switchover in the event of a primary controller failure. This means that critical processes
are not affected by control system hardware failures. The results are: an increased productivity, a minimized downtime and
low maintenance times.
The communication between the controllers is established at the end of each CPU cycle through two high-speed redundancy
links.
Figure 13: CPU Redundancy
At the center of a two half cluster redundant system, there is a pair of redundant CPUs. The redundancy links – between
the two CPUs – have two channels, so single failures on each channel will not affect system performance.
The Active CPU executes the application program and controls the remote I/O. The Standby CPU is put as a background
resource, ready to take over if necessary. The standby CPU is connected to the active CPU via a high-speed link present on
a module called Redundancy Link Module. This module must be placed at right side of the CPU. The CPU and Redundancy
Link module must be placed in certain slots in the racks. For details, check the CPU user manual. In the event of an unexpected
failure affecting the active CPU, the standby system switches over automatically, changing execution of the application program
and control of the I/O over to the standby CPU, with an up-to-date data context. Once they had switched over, the standby
CPU becomes the active CPU.
Configuration of both systems shall be identical. CPU modules will be placed at identical slots in both systems. After
power on, one of the CPUs operates as active and another CPU enters standby mode. Active CPU will update the system status
of standby CPU at the end of every scan. So the standby CPU is always updated with latest I/O status and results of program
execution in the active CPU. This application is easy to set up, with no special programming or parameterization needed by
the user.
1.1.4.10.
Minimum Configuration of a Redundant PLC (Without PX2612 Panel Usage)
The redundant PLC is composed at least by two half-clusters identically, where each half-cluster is formed of the following
modules:
rack where the modules are inserted, and can be NX9000, NX9001, NX9002 and NX9003
power supply NX8000
the CPU NX3030
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