# Clocking.HBus.ClockSource

Specifies the clock reference origin.

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.ClockSource = clock_source

#### Access

Read/Write

#### Type

String

#### Default

OSC

#### Allowed values

OSC | A_CLOCK | B_CLOCK | NETWORK | NETREF

#### Example

Clocking.HBus.ClockSource = OSC

#### Details

Valid entries for the keyword include:

Value | Description
--- | ---
OSC | Uses the on-board oscillator as a reference.
A_CLOCK | Causes the board to act as a clock slave to the H.100 bus A clock by deriving the local clock from the bus. Another H.100 board (or H.110 board) must drive the clock on the bus.
B_CLOCK | Causes the board to act as a clock slave to the H.100 bus B clock by deriving the local clock from the bus. Another H.100 board (or H.110 board) must drive the clock on the bus.
NETWORK | Causes the board to derive the local clock, telephony bus clock, and line transmit clock using the clock extracted from one of the BRI trunks.
NETREF | H.100 network reference.