# Clocking.HBus.ClockSource

Specifies the timing reference for the board to use based on the Clocking.HBus.ClockMode setting.

For more information about setting up CT bus clocking and about the rules and restrictions that apply to setting up clocking with CG 6565 boards, 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 | NETREF | NETWORK

#### Example

Clocking.HBus.ClockSource = NETWORK

#### Details

Valid entries include:

Value | Description
--- | ---
OSC | Drives 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 first timing reference.
B_CLOCK | Uses CT_C8_B and CT_FRAME_B timing signals on the H.100 bus.
NETREF | Uses NETREF as the board's first timing reference. The NETREF reference source is set with Clocking.HBus.NetRefSource. The source may be a trunk on another board.
NETWORK | Uses a digital trunk as the board's first timing reference. The trunk must be specified with the Clocking.HBus.ClockSourceNetwork keyword.

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