Allen-Bradley ControlNet PLC-5, Руководство пользователя

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Publication 1785-6.5.24 February 1999
1-4 Backup Concepts for the ControlNet PLC-5 Backup System
Bumpless Transfer
Ideally, when a processor switchover takes place in the backup
system, there should be no adverse effect on system operation, and
the controlled process should continue to run as though it were still
being controlled by the original primary processor. This is generally
referred to as a bumpless transfer of control.
Bumpless transfer is not required for many backup applications, but it
is highly desirable. Divergence in program scanning, I/O tables, data
tables, and long switchover times can all cause bumps when
transferring control from a primary to a secondary processor. The
ControlNet PLC-5 backup system allows you to choose synchronous
program scan operation, as well on-demand crossloading of data table
files you select. These options can greatly reduce the possibility of
bumps during a processor switchover.
Switchover
Switchover is what occurs when the secondary processor assumes
control of the process, thus becoming the primary processor. Several
conditions can cause this to happen:
• critical failures in the primary processor (e.g., hardware faults)
• internal conditions within the primary processor (e.g., a major
fault), causing it to relinquish control of the process to the sec-
ondary processor
• external conditions recognized by the secondary processor (e.g.,
loss of communications with the primary processor) causing it to
assume control of the process
• a manual switchover (i.e., a forced switchover, where you cause
the secondary processor to become primary, and vice versa; you
may choose to do this when testing your system for backup
performance and integrity, or as part of a system maintenance
program).
The ideal switchover time from the primary processor to the
secondary processor would be instantaneous. The ControlNet PLC-5
backup system typically provides a switchover time of 30-50 ms.
This time is adequate to avoid a bump in most systems, but you
should determine the maximum allowable switchover time for your
applications.