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4-2 Series 90™-70 Enhanced Hot Standby CPU Redundancy User's Guide
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May 2000 GFK-1527A
4
Powerup of a Redundant CPU
When a redundant CPU is powered up, it performs a complete hardware diagnostic check and a
complete check of the application program and configuration parameters. This causes the powerup
time of a redundant CPU to be significantly longer than the normal powerup time of a non-
redundant CPU. If the Primary and Secondary systems power up together each CPU will recognize
this fact so that the Primary system will become the active and the Secondary system the backup.
Powerup consists of the following sequence of steps:
1. Powerup self-test is always performed.
2. CPU operating system is initialized and PLC memory is validated.
3. Diagnostics called during full powerup tests are performed.
4. System Configuration is verified.
5. System is interrogated and initialized.
6. Presence of other CPU is detected.
7. Redundancy Communications Modules are initialized.
8. Complete application program is verified.
9. CPU synchronizes with redundant CPU.
When the Secondary Unit powers up, if it does not detect the Primary Unit, the Secondary Unit
waits up to 15 seconds for the Primary Unit to power up. If the primary unit has not completed its
powerup sequence within 15 seconds, the Secondary Unit assumes the Primary Unit is not present.
If at this time, the Secondary Unit transitions to RUN mode, it does so as an active unit without a
backup unit.
If the Primary Unit completes its powerup sequence before the Secondary Unit, the Primary Unit
does not wait for the Secondary unit to complete its powerup sequence. If the Primary Unit is set
up to transition to RUN on powerup (that is, was powered-down in RUN mode), it transitions to a
stand-alone unit without waiting for the Secondary unit. The Secondary Unit, upon completion of
its powerup sequence, establishes communications with the Primary Unit. If transitioning to Run
mode, it synchronizes with the Primary Unit.
In either case, if one CPU fails to notify the other CPU that it is either present or powering up, the
other CPU, if transitioning to RUN, becomes the active unit and runs without a backup unit.
Resynchronization occurs after the powerup sequence is complete.
Note
If the system should be fully redundant upon powerup, the Secondary Unit must
complete power-up first but no more than 15 seconds before the Primary Unit.
The way to be sure this happens is to apply power to the Secondary Unit first.