4.4.  Required Clock Designations

• 4.4.1.  Clocking Examples
• Synchronous Clocking Example
• Node 1: SCxbus Master Clock
• Plesiochronous Clocking Example

The following clock designations are required in an SCxbus multi-node system.

For the SCxbus Master Clock Node, make the following selections:

• Select the Network Interface Board that will provide SCbus Master Clock
• Select any remaining Network Interface Board(s) and/or the SCX160 SCxbus Adapter to be used as an alternate fallback clock for the SCbus Master Clock source and place these boards into a preferred fallback order. The SCX160 SCxbus Adapter should be last.
• For the SCX160 SCxbus Adapter:
  • Designate slave mode for the SCbus interface
  • Designate Master Clock for the SCxbus interface
  • Designate a reference clock source for the SCX160 SCxbus Adapter, typically, the SCbus

For the SCxbus Fallback Master Clock Node, make the following selections:

• If a Network Interface Board is available in this node to provide SCbus Master Clock, then designate this board as the source of SCbus Master Clock
• If additional Network Interface Boards are available in this node that can provide fallback clock for the SCbus Master Clock source, then designate these boards as a fallback SCbus Master Clock source and place these boards into a preferred fallback order
• For the SCX160 SCxbus Adapter:
  • If the source of SCbus Master Clock is a Network Interface Board as described above, then designate slave mode for the SCbus interface
  • If the source of SCbus Master Clock is the SCX160 SCxbus Adapter (no Network Interface Board in this node is designated to provide SCbus Master Clock), then designate Master Clock Mode for the SCbus interface
  • Designate Fallback Master Clock for the SCxbus interface
  • Designate a reference clock source for the SCX160 SCxbus Adapter, typically, the SCbus

For all other SCxbus Nodes/SCX160 SCxbus Adapters, make the following selections:

• For synchronous clocking:
  • Designate Synchronous Slave mode for the SCxbus interface
  • Designate Master Clock mode for the SCbus interface
  • Designate a reference clock source for the SCX160 SCxbus Adapter, typically, the SCxbus
• For plesiochronous clocking:
  • Designate Plesiochronous Slave mode for the SCxbus interface
  • Designate slave mode for the SCbus interface

4.4.1.  Clocking Examples

The following examples illustrate the interactions between the designated Master Clock node, the Fallback Master Clock node and the slave nodes in both a synchronous and a plesiochronous clocked system.

Synchronous Clocking Example

Figure 6.  Synchronous Clocking Example provides an illustration of synchronous clocking for a multi-node system.

While referring to the figure, assume the following:

• Node 1 is designated as the source of SCxbus Master Clock
• Node 2 is a fallback node designated as the source of SCxbus fallback Master Clock
• Nodes 3 through 16 are designated as Synchronous Slave Mode nodes

The remainder of this section describes the operation of the nodes in the example.

Figure 6.  Synchronous Clocking Example

Node 1: SCxbus Master Clock

Node 1 includes one or more Network Interface Boards that provide a System Wide Clock Master (SWCM) reference for the entire multi-node system. This SWCM drives the SCbus Master Clock signal which is applied to the SCX160 SCxbus Adapter SCbus interface. In this configuration, the SCX160 SCxbus Adapter SCbus interface operates in Slave Clock Mode.

The SCX160 SCxbus Adapter uses the SCbus master clock as the clock source for the SCxbus and thus for the entire multi-node system by operating its SCxbus interface in SCxbus Master Clock Mode.

For this example, the SCX160 SCxbus Adapter synchronizes the multi-node system with the Right SCxbus Clock and Frame Synchronization signal. This clock reference is sent to all other SCX160 SCxbus Adapters via the SCxbus.

Node 2: SCxbus Fallback Master Clock

In this example, Node 2 is designated as the fallback node. In this configuration, the SCX160 SCxbus Adapter SCxbus interface operates in SCxbus Fallback Master Clock mode.

The SCX160 SCxbus Adapter uses the Right SCxbus Clock and Frame Synchronization signal from Node 1 to phase synchronize the Left SCxbus Clock and Frame Synchronization signal. This signal is then sent to all other SCX160 SCxbus Adapters via the SCxbus to be used if the Master Clock from Node 1 fails.

The SCX160 SCxbus Adapter SCbus interface can operate in either of the following modes:

• Master Clock Mode (synchronous operation)
• Slave Clock Mode (plesiochronous operation)

To have a fully synchronized system where all nodes and SCSA boards clock from the master clock reference in the master clock node (node 1 in this example), the SCbus interface of the node 2 SCX160 SCxbus Adapter would be set to Master Clock Mode. In this mode, the SCX160 SCxbus Adapter provides the SCbus master clock to drive all boards connected to the SCbus.

For the fallback master clock node (node 2 in this example), plesiochronous operation may be more advantageous. If the Right SCxbus Clock is lost, then all slaved nodes automatically switch to the Left SCxbus Clock for their clock reference. The fallback master clock node (node 2) will switch to an application designated reference clock selected from one of the following, if available:

• SCbus clock
• Local Crystal Oscillato
• 8 kHz external reference clock from the SCxbus

If the designated reference clock is not available, then the fallback node automatically switches to its Local Crystal Oscillator.

Typically, the most stable clock reference would be clock from a locally installed network board. If a network board is providing SCbus master clock, then the SCX160 SCxbus Adapter can fallback to the SCbus master clock as its reference clock and continue to provide a highly stable clock to the SCxbus network. In this configuration, the SCbus interface of the SCX160 SCxbus Adapter operates in slave clock mode so that the SCbus master clock is always available to the SCX160 SCxbus Adapter.

Nodes 3 to 16: Synchronous Slave Mode

In this example, these nodes use the master clock reference from the SCxbus. In this configuration, the SCX160 SCxbus Adapter SCxbus interface operates in Synchronous Slave Mode.

The SCX160 SCxbus Adapter uses Right SCxbus Clock and Frame Synchronization signal from Node 1 as its master clock source. If the Master Clock in Node 1 fails, the SCX160 SCxbus Adapter automatically falls back to the Left SCxbus Clock and Frame Synchronization signal from Node 2.

The SCX160 SCxbus Adapter SCbus interface in these nodes is set to operate in Master Clock Mode so that all SCSA boards installed in these nodes are clocked from the SCX160 SCxbus Adapter.

Plesiochronous Clocking Example

Figure 7.  Plesiochronous Clocking Example provides an illustration of plesiochronous clocking in a multi-node system.

While referring to the figure, assume the following:

• Node 1 is designated as the source of SCxbus Master Clock
• Node 2 is a fallback node designated as the source of SCxbus fallback Master Clock
• Nodes 3 through 16 are designated as Plesiochronous Slave Mode nodes

The remainder of this section describes the operation of the nodes in the example.

Node 1: SCxbus Master Clock

Node 1 includes one or more Network Interface Boards that provide a SWCM reference for the entire multi-node system. This SWCM drives the SCbus Master Clock signal which is applied to the SCX160 SCxbus Adapter SCbus interface. In this configuration, the SCX160 SCxbus Adapter SCbus interface operates in Slave Clock Mode.

The SCX160 SCxbus Adapter uses the SCbus master clock as the clock source for the SCxbus and thus for all SCX160 SCxbus Adapters in the multi-node system by operating its SCxbus interface in SCxbus Master Clock Mode.

For this example, the SCX160 SCxbus Adapter synchronizes all SCX160 SCxbus Adapters in the multi-node system with the Right SCxbus Clock and Frame Synchronization signal via the SCxbus.

Node 2: SCxbus Fallback Master Clock

In this example, Node 2 is designated as the fallback node. In this configuration, the SCX160 SCxbus Adapter SCxbus interface operates in SCxbus Fallback Master Clock Mode.

The SCX160 SCxbus Adapter uses the Right SCxbus Clock and Frame Synchronization signal from Node 1 to phase synchronize the Left SCxbus Clock and Frame Synchronization signal. This signal is then sent to all other SCX160 SCxbus Adapters via the SCxbus to be used if the Master Clock from Node 1 fails.

The SCX160 SCxbus Adapter SCbus interface is set to operate in Slave Clock Mode (plesiochronous operation).

Figure 7.  Plesiochronous Clocking Example

If the Right SCxbus Clock is lost, the fallback master clock node (node 2) will switch to an application designated reference clock selected from one of the following, if available:

• SCbus clock
• Local Crystal Oscillator
• 8 kHz external reference clock from the SCxbus

If the designated reference clock is not available, then the fallback node automatically switches to its Local Crystal Oscillator.

Typically, the most stable clock reference would be clock from a locally installed network board. If a network board is providing SCbus master clock, then the SCX160 SCxbus Adapter can fallback to the SCbus master clock as its reference clock and continue to provide a highly stable master clock to the SCxbus network. In this configuration, the SCbus interface of the SCX160 SCxbus Adapter operates in slave clock mode so that the SCbus master clock is always available to the SCX160 SCxbus Adapter.

Nodes 3 to 16: Plesiochronous Slave Mode

In this example, the SCX160 SCxbus Adapter in each node uses the master clock reference from the SCxbus to synchronize SCxbus activities. In this configuration, the SCX160 SCxbus Adapter SCxbus interface operates in Plesiochronous Slave Mode.

The SCX160 SCxbus Adapter uses Right SCxbus Clock and Frame Synchronization signal from Node 1 as its master clock source. If the Master Clock in Node 1 fails, the SCX160 SCxbus Adapter automatically falls back to the Left SCxbus Clock and Frame Synchronization signal from Node 2.

The SCX160 SCxbus Adapter SCbus interface in these nodes is set to operate in Slave Clock Mode so that the SCX160 SCxbus Adapter SCbus interface and all SCSA boards installed in the node are clocked from the SCbus.

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