5700ACO
Automatic Changeover
Page 26 Revision 1.0
There are three automatic modes of operation:
1. One Change Mode: Switch one time only on the first valid switch criteria.
2. Dual Change Mode: Suspend all switching when two consecutive criteria are met within a short
time period.
3. Unlimited Change Mode: Switch whenever a valid criterion is met.
The selection of the above modes will depend on the desired "change over" behavior. Sections
4.2.1 to
4.2.3 provide descriptions for these modes.
4.2.1. One Change Mode
The user may want the ACO to switch to the backup unit and stay there until you diagnose the issue. In
this case, use the "one change" mode. When an automatic changeover switch occurs, the backup unit
is selected and all further changeovers are suspended. The Auto LED will flash to indicate a
changeover and that attention is needed to troubleshoot the problem. The ACO will have to be re-
armed after the problem is resolved; otherwise, a failure on the active system will not trigger a switch.
To re-arm the auto changeover logic, move the CHANGEOVER MODE switch to Manual mode and
back to Auto mode. Notice that the Auto LED is now on solid, indicating an armed situation.
4.2.2. Dual Change Mode
The signal paths through the ACO are completely passive except for mechanical relays that will
maintain connections, even with the power to the ACO turned off. There are no active components in
the signal path, and the input circuitry that is monitoring the inputs (for decision making) is high
impedance. This is a good architecture except for one problem: If a signal output of the ACO is shorted
to something it should not be shorted to (i.e. ground), then the decision logic will fail. Consider the
following:
1. Input A is the selected path.
2. Someone shorts an output of the ACO to ground.
3. This short kills the signal as detected on input A, but input B is fine since it is not connected to
the short circuit.
4. The ACO sees the missing signal and switches to input B.
5. Input B is now connected to the shorted output.
6. Input A is now isolated from the shorted output.
7. The ACO sees a missing signal on the B input and a good signal on the A input so it switches to
A.
8. The ACO will repeat this condition creating an oscillating A/B switching behavior.
To address this issue we have designed a number of features to minimize system impact in the event
of an output short. One item is a mode that will detect two consecutive changeovers within a short
period of time. If this happens, further changes are suspended and the Auto LED will flash. This gives
you time to diagnose the situation. Once you are satisfied that you have rectified the problem, re-arm
the auto changeover logic, by moving the CHANGEOVER MODE switch to Manual mode and back to
Auto mode.
Multiple changeover events, separated by more than approximately half a second are considered valid
conditions for ACO operation. This is the recommended operating mode because it offers normal
operation while minimizing system impact in the event of an output short.
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