4.4. Designating the Clock Source

The DCM automatically determines the bus mode of the system based on the TDM bus capabilities of installed devices: for H.100 capable boards, the bus mode is H.100; for H.110 capable boards, the bus mode is H.110; for SCbus capable boards, the bus mode is SCbus.

Note: In the current system software release, any system that contains an SCbus-only board must operate in SCbus mode.

The DCM selects default clock source settings for H.100 and H.110 systems as follows:

• The DCM automatically selects the primary master, as indicated by the Primary Master FRU (Resolved) parameter.
• If the primary master is capable of deriving the clock signal from a CT Bus network reference signal, then the default clock source will be the output signal from the network reference board. The NETREF One FRU (Resolved) parameter indicates the network reference board.

  Note: In the current release, the network reference board will be the same board as the primary master (that is, the value of the Primary Master FRU (Resolved) parameter will be the same as the value of the NETREF One FRU (Resolved) parameter).

• If the primary master is not capable of deriving the clock signal from a CT Bus network reference signal, then the default clock source will be the first network interface trunk on the primary master board. (The network reference board derives its signal from the first network interface trunk. This is indicated by the Derive NETREF One From (Resolved) parameter.)

Refer to Section 4.4.1, Setting the Clock Source for H.100 and H.110 Mode Systems for information about changing the default clock source settings for H.100 and H.110 systems

In addition, refer to "Section 2.1.4, "Configuring a Mixed CT Bus/SCbus System", on page 9" for information about mixed systems and refer to "Section 2.1.5, "Understanding the CT Bus", on page 13" for general information about H.100/H.110 CT Bus concepts.

The DCM selects default clock source settings for SCbus systems as follows:

• The DCM automatically selects the primary master, as indicated by the Primary Master FRU (Resolved) parameter.
• The default clock source is the internal oscillator of the primary master, as indicated by the Derive Primary Clock From (Resolved) parameter. However, the internal oscillator should only be used as the clock source in the following cases:
  • Internal testing purposes
  • If the primary master is an analog board without a digital network interface.
  • If the primary master is a resource-only board without a digital network interface.

You should set the system to derive clocking from a digital network trunk that is connected to the primary master's front end. Refer to Section 4.4.2, Setting the Clock Source for SCbus Systems for information about changing the default clock source settings for SCbus systems.

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