# Configuring the primary clock master

The primary clock master drives a CT bus clock used as the primary timing reference by all other boards connected to the CT bus. Use the following keywords to configure the primary clock master:

Keyword | Description
--- | ---
Clocking.HBus.ClockMode | Specifies the CT bus clock that the board drives. For the primary clock master, specify: MASTER_A for A_CLOCK MASTER_B for B_CLOCK
Clocking.HBus.ClockSource | Specifies the primary timing reference for the board. For the primary clock master, set to any of the following: NETREF to use NETREF (also called NETREF1) NETREF2 to use NETREF2 (H.110 only) NETWORK to derive the timing from the clock pulse on a digital trunk connected to the board OSC to use the board's on-board oscillator. Use only when no other source is available.
Clocking.HBus.ClockSourceNetwork | If Clocking.HBus.ClockSource is set to NETWORK, specifies the board trunk to derive the primary timing reference from (1 to n, where n is the number of trunks on the board).
Clocking.HBus.AutoFallBack | Enables or disables clock fallback on the board. When set to YES, specifies that the board automatically switches to the Clocking.HBus.FallBackClockSource timing reference when the Clocking.HBus.ClockSource timing reference fails. The board continues to drive the CT bus clock using this timing reference until the first timing reference is re-established.
Clocking.HBus.FallBackClockSource | Specifies the fallback timing reference for the board to use if the primary timing reference fails. The board continues to drive the CT bus clock using this timing reference until the primary timing reference is re-established. For the primary clock master, set to any of the following: NETREF to use NETREF (also called NETREF1) NETREF2 to use NETREF2 (H.110 only) NETWORK to derive the timing from the clock pulse on a digital trunk connected to the board OSC to use the board's on-board oscillator. Use only when no other source is available. Note: The fallback timing reference must be different from the primary timing reference.
Clocking.HBus.FallBackNetwork | If Clocking.HBus.FallBackClockSource is set to NETWORK, specifies the board trunk from which to derive the fallback timing reference. (1 to n, where n is the number of trunks on the board).
Clocking.HBus.NetRefSpeed | If using NETREF(1) or NETREF2 as a timing reference, set to 8K.