# Clocking.HBus.FallBackClockSource

Specifies the alternate timing reference to use when the first timing reference does not function properly.

For more information about setting up CT bus clocking and about the rules and restrictions that apply to setting up clocking with CG 6565E boards, refer to Configuring board clocking.

#### Syntax

Clocking.HBus.FallBackClockSource = clock_source

#### Access

Read/Write

#### Type

String

#### Default

OSC

Fallback to OSC is not recommended because the transition can cause slave boards to fall back to the secondary clock and create an out-of-sync condition.

#### Allowed values

OSC | A_CLOCK | B_CLOCK | NETREF | NETWORK

#### Example

Clocking.HBus.FallBackClockSource = B_CLOCK

#### Details

Valid entries include the following values:

Value | Description
--- | ---
OSC | Causes the board to drive the T1 or E1 line transmit clock using the on-board oscillator. Use this setting only when the board's T1 or E1 connection is isolated from the network. Apply this setting, for example, if you use a T1 or E1 connection as a link between two computers, or if you use one board to simulate network traffic to another. The on-board oscillator is accurate to 32 ppm (parts per million) and meets the requirements for a Stratum 4E clock.
A_CLOCK | Uses CT_C8_A and CT_FRAME_A timing signals as the board's fallback timing reference.
B_CLOCK | Uses the CT_C8_B and CT_FRAME_B timing signals as the board's fallback timing reference.
NETREF | Uses NETREF as the board's fallback timing reference. The NETREF reference source is set with Clocking.HBus.NetRefSource. The source can be a trunk on another board.
NETWORK | Uses a digital trunk as the board's fallback timing reference. This trunk is specified with the Clocking.HBus.ClockSourceNetwork keyword.

When this keyword is set to NETWORK, you must also specify the fallback network timing reference source with the Clocking.HBus.FallBackNetwork keyword.

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

#### See also

Clocking.HBus.ClockMode, Clocking.HBus.ClockSource