# Clocking.HBus.FallbackClockSource

Specifies the secondary clock reference to use when the primary clock reference fails.

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.FallbackClockSource = argument

#### Access

Read/Write

#### Type

String

#### Default

OSC

#### Allowed values

OSC | A_CLOCK | B_CLOCK | NETREF

#### Example

Clocking.HBus.FallBackClockSource = OSC

#### Details

If the Clocking.HBus.AutoFallBack keyword is set to NO, this keyword is ignored.

Valid entries for this keyword are:

Value | Description
--- | ---
OSC | Valid only if the board is a clock master. OSC causes the board to use its on-board oscillator as its secondary timing reference. Use this setting only when no external timing reference is available. The on-board oscillator is accurate to 32 ppm (parts per million) and meets the requirements for a Stratum 4E clock.
A_CLOCK | Use the setting if the board is a clock slave to B_CLOCK, and a secondary clock master is driving A_CLOCK. This setting causes the board to use A_CLOCK as its secondary timing reference.
B_CLOCK | Use the setting if the board is a clock slave to A_CLOCK, and a secondary clock master is driving B_CLOCK. This setting causes the board to use B_CLOCK as its secondary timing reference.
NETREF | Valid only if the board is a clock master. NETREF causes the board to use the signal from the NETREF carrier on the CT bus as its secondary timing reference. Another source is driving NETREF. This source is specified using the Clocking.HBus.NetRefSource keyword.

The board returns an error if you select a CT bus clock source and no source is detected.

For more information about clock fallback, refer to the NMS OAM System User's Manual.

#### See also

Clocking.HBus.ClockMode, Clocking.HBus.ClockSource