Conditions for Switchover

Commanding Manual Switchovers

In addition to the system conditions that cause an automatic Switchover, a manual Switchover can be commanded by:

writing to bits 1 and 2 of the Command Register at %SW60.

This write operation can be accomplished by:
- the application
- issuing a Modbus request from a remote HMI
- Control Expert animation table
sending a RUN -> STOP command from Control Expert to the Primary CPU
Offline command from the Primary CPU keypad

NOTE: Before doing any switchover by application program, ensure that the standby PLC is ready to assume the Primary role. Refer to the chapter Quantum-specific System Words for more information about the %SW182-%SW183 and %SW176-%SW177 system words.

NOTE: The intended use of user application Switchover (in %SW60) is to react to. detected error by the application. Do not use this method for periodic Switchovers.

NOTE: If for some reason the application has to Switchover periodically, the period between switchovers must not be less 120sconds.

Example of Switchover with PLC B Initially in Standby Mode

In this example, the initial state of the system is as follows:

PLC A has a RUN command (%SW60.1 = 1) and is acting as the Primary
PLC B has a RUN command (%SW60.2 = 1) and is acting as the standby

By writing new values to bits 1 and 2 of the %SW60 command register, you can command a change in the operating modes of the Hot Standby controllers. The following table describes the four commands and their results:

New Values Written to %SW60		Resulting PLC Operating Modes		Effects
Bit 1	Bit 2	PLC A	PLC B	Effects
0	0	Offline	Offline	System is not controlling the application
0	1	Offline	Standby ↓ Primary	Switchover event occurs within one MAST task* System is no longer redundant
1	0	Primary	Offline	No Switchover event System is no longer redundant
1	1	Primary	Standby	No Switchover event No change from initial conditions
* In this case, we are not directly commanding a Switchover. Instead, we are commanding PLC A to enter an Offline state and we are relying on the system logic to recognize this and change PLC B from standby to Primary during the next scan.

NOTE: All changes to the command register %SW60 must be written to the Primary PLC. This register is copied from the Primary to the standby PLC during each MAST task. Therefore, any changes you make directly to the standby PLC's command register will be overwritten by this transfer without taking effect.

Example of Switchover not Possible

In the event of a detected error in the Primary CPU and if the mode of the standby CPU is Offline, the standby CPU is not ready to assume the role of Primary CPU. No switchover is possible.