# Clocking.HBus.AutoFallBack

Enables or disables clock fallback on the board.

For information about setting up CT bus clocking, and rules and restrictions for configuring CT bus clocking, refer to Configuring board clocking.

#### Syntax

Clocking.HBus.AutoFallBack = mode

#### Access

Read/Write

#### Type

String

#### Default

NO

#### Allowed values

YES | NO

#### Example

Clocking.HBus.AutoFallBack = YES

#### Details

When set to YES, this keyword specifies whether or not the board automatically switches between the two clock timing references specified by the Clocking.HBus.ClockSource and Clocking.HBus.FallBackClockSource keywords. The Clocking.HBus.AutoFallBack keyword applies for all modes specified by the Clocking.HBus.ClockMode keyword.

The fallback timing reference clock is selected by the Clocking.HBus.FallBackClockSource keyword. Both of the physical timing references specified by the Clocking.HBus.ClockSource and Clocking.HBus.FallBackClockSource keywords must be present and not in alarm when the board's clocking is set up.

NO indicates that the system does not fallback to the backup timing reference.

Specify the primary clock and fallback clock with the Clocking.HBus.ClockSource and Clocking.HBus.FallBackClockSource keywords.

If the board is configured as the primary master or in standalone mode, this keyword enables the board to switch to the secondary timing reference when the first source goes into an alarm state. If the primary source returns, the board's timing reference switches back to the primary source. The showclks utility program can be used to determine what timing reference the board is actively using.

For an AG board configured as a secondary clock master or as a clock slave, this keyword enables the board to switch to an alternative timing reference when the first source goes into an alarm state. The board does not return to the first timing reference if the timing reference recovers. The host application must perform any further clock configuration operations.

For more information about clock fallback, refer to the Switching Service Developer's Reference Manual.

To support clock fallback on an AG board, refer to the NMS web site (www.nmscommunications.com) for application notes and other updates.