9.537 Protection-Switch-Asymmetric

The Protection-Switch-Asymmetric logical device provides a software based solution for a 1:1 redundancy switch. As the name suggests, the device is asymmetric, it differentiates between a primary and a backup signal chain. In simple terms, the device switches to backup when there is a fault at the primary chain and it switches back to primary when the primary chain becomes healthy again.

In practice, things are a bit more sophisticated. Beside the primary chain fault there are other conditions which are to be taken into account, also there are a number of delay parameters which make the protection switch behave in a useful way.

Fault monitoring

The logical device monitors the fault states of a couple of devices, deriving from this the information whether to switch over. Actually the device supports five fault flags for each chain which are logically ORed to obtain the chain fault.

Inhibit signals

The device never switches to a chain which states a fault. Beside this, the device can monitor an additional fault flag for each chain which is interpreted as an inhibit signal. If this flag is true, the device will not switch to this chain, even if the opposite chain states a fault.

Delay pararameters

The Protection-Switch-Asymmetric logical device provides three delay parameters which control the device's behaviour in different situations.

• Switchdelay --- This delay controls the time, a fault condition must be present before the device switches from primary to backup
• Recoverydelay --- If the device switches to backup, the primary chain states OK again and a fault happens at the backup chain, the device switches back to the primary chain after this time. This delay prevents the redundancy switch from oscillating if the switch over process itself causes a short fault at the backup chain.
• Switchbackdelay --- This delay controls the time, after that the device switches back to primary when no more faults are active. This time usually is set to a much longer value than the recovery delay.

All delay values may be set to zero resulting in an immediate reaction of the device.

Switchover conditions

With protection enabled, the device switches from primary to backup if the backup chain shows no fault and no inhibit condition, but the primary chain signals a fault and this condition stays true for at least 'switch delay' seconds.

The device switches back from backup to primary if the primary chain shows no fault and no inhibit condition, but the backup chain signals a fault and this condition stays true for at least 'recovery delay' seconds.

The device also switches back from backup to primary if the primary chain shows no fault and no inhibit condition, regardless of the state of the backup chain with this condition being true for at least 'switch back delay' seconds.

Device window pages

The following table shows which device window pages are available with this individual device type. Tool-bar functions not mentioned here are described at the general description of device windows .

• --- This page controls the operational parameters of the device.
• --- The faults page. * --- The device info page.
• --- The maintenance page. The configuration parameters described below are set at this page.

Configuration parameters

At the maintenance page of the device window there are a couple of configuration parameter which must be set to interconnect the redundancy switch with the devices it shall monitor and control.

Variables defined by this device driver