# Clocking.HBus.ClockSource

Specifies the primary timing reference for 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.ClockSource = argument

#### Access

Read/Write

#### Type

String

#### Default

OSC

#### Allowed values

OSC | A_CLOCK | B_CLOCK | NETREF

#### Example

Clocking.HBus.ClockSource = OSC

#### Details

Valid entries for this keyword are:

Value | Description
--- | ---
OSC | Valid only if the board is the primary clock master or in standalone mode. OSC causes the board to drive the bus clock using the signal from its on-board oscillator. 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 | Valid only if the board is a clock slave or secondary master. This setting causes the board to act as a slave to A_CLOCK.
B_CLOCK | Valid only if the board is a clock slave or secondary master. This setting causes the board to act as a slave to B_CLOCK.
NETREF | Valid only if the board is the primary clock master. NETREF causes the board to drive the bus clock using a signal from the NETREF carrier on the CT bus. 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 clocking, refer to the NMS OAM System User's Manual.

#### See also

Clocking.HBus.AutoFallBack, Clocking.HBus.ClockMode, Clocking.HBus.FallbackClockSource