The DeltaV controller constantly scans both the active and standby cards and directs a switchover from the active to the standby card when a failure occurs. Switchover is very smooth and occurs in a matter of milliseconds. The last known good values of the output are held during switchover. A redundancy switchover can occur under the following circumstances:
Open and short circuit faults in the channels during installation or operation
Card errors due to external noise near the system or noise on the carriers, cards, or channels
Improper grounding that might cause some channels to draw huge currents and high voltages
Momentary loss of field power
Fatal condition on the active card (for example, the active card cannot communicate with the controller)
Non-fatal condition on the active card. These conditions are dependent upon the card type (for example, a failure of the A/D converter on an AI card)