4-22 Series 90™-70 Enhanced Hot Standby CPU Redundancy User's Guide – May 2000 GFK-1527A
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a scan set that is scanned every other sweep (that is, PERIOD=2), then th e Primary CPU might
scan its scan set in one sweep and the S econ dary CPU scan its scan set in the next.
Use of non-default scan sets can ca use variance in the time the units get t o the rendezvous points.
This should be considered when determining the failwait time.
C Debugger
The Embedded C debugger may be used for debugging Standalone C programs and EXE blocks.
Use of the embedded C debugger in a Redundancy CPU is limited to when the system is not
synchronized. The CPU will reject any attempt to establish a debugger session while the units are
synchronized. If the debugger is active on one unit while the two units are not synchronized, then
any attempt to synchronize the two units will fail. Specifically, if the unit in RUN mode has a
debugger session active and the other unit is commanded to go to RUN mode, the unit commanded
to go to RUN will log a fault and go to STOP/FAULT mode.
STOP to RUN Mode Transition
A resynchronization will occur at all STOP to RUN mode transitions. The time to p erform this
resynchronization may be larger than STOP to RUN transitions on non-redundancy CPUs. The
STOP to RUN mode transition has two separate paths.
1. If the CPU performing the transition is doing so alone or both CPUs are transitioning at the
same time, then a normal STOP to RUN mode transition is performed (clear non-retentive
memory and initialize FST_SCN and FST_EXE).
2. If the other CPU is active when this CPU performs a STOP to RUN mode transition, then non-
retentive references will be cleared followed by a resynchronization with the active CPU.
Background Window Time
In a redundancy system, this value may be set to zero. Unlike other CPU models which have a
default of 0mS, the default value for the Redundancy CPU is 5ms.
Setting the background window time to zero disables the verification of the Series 90-70 CPU
operating system software an d the CPU self-tests.
Sequential Function Chart Programming (SFC)
SFC Program Block s c a n be used in the prog ram logic. However , the redundant CPU s ystem will
not attempt to coordinate and synchronize the execution of the SFC charts between the two CPUs.
For example, if one of the units is in Run mode at the time the other is placed in Run mode, the
running unit will typically be in the middle of its chart, and the transitioning unit will typically be
at the beginning of its chart. As a result, the SFC state and paths taken by the two CPUs will be
different and the backup unit will not be able to take over exactly where the active unit left off.