2 – Theory of Operation
T1 CrossConnect Installation & Operation Manual
Version 2
GatesAir
2-3
Intraplex Products
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If a problem occurs with the primary clock source and the DCS software switches to the alternate
clock source, the software does not attempt to switch back if the problem clears unless there is a
problem with the alternate.
2.1.5.5 System Clock Switchovers
Various software functions monitor the state of each clock source. The DCS software switches the
primary and alternate clock sources based on specific determining factors. Switchovers occur for these
reasons:
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A DCS user requests a change in the clock source configuration by means of the
::SET:CLOCK
command. This command executes a clock switchover only if all of these conditions are met:
•
The requested clock source is different from the current source for the system clock.
•
The requested clock source is in service.
•
The hardware successfully latches in the requested clock source.
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The primary clock source (the clock-in-use) has failed or is no longer present. For example, the T1
port driving the system clock may have failed. This forces a switchover to the alternate clock
source, or the internal oscillator, if no value for an alternate clock source is expressed in the
::SET:CLOCK
command.
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The DCS software detects in the primary clock source the occurrence of excessive clock slips (3 or
more) in the same direction. This forces a switchover to the alternate clock source or the internal
oscillator, if no alternate has been set.
Clock slippage
is the detection of two clock waveforms out of synchronization. A slip detect circuit
compares the waveform of the actual clock to the waveform of the reference clock. If the waveforms
differ by three or more T1 bit intervals in the same direction, a switchover occurs from the primary
clock to the alternate clock. The switchover causes the alternate clock to become the clock-in-use (the
new system clock) and the previous primary clock to become the new alternate clock.
2.1.5.6 Clock Switchover Occurrence
These two examples show what happens when switchovers occur:
Example 1:
Consider a state in which the source of the system clock (the actual clock or clock-in-use) is the
internal oscillator. You decide to change this source to port 2 and use ISiCL to execute the command.
The events for Example 1 are
1.
A user executes
<address>:DCS:SET:CLOCK=2,4;
which specifies that the primary clock source is port 2 and the alternate clock source is port 4. The
source of the requested clock is port 2, although the actual clock source continues to be the
internal oscillator at this time.
2.
The source of the requested clock also becomes the source of the reference clock (port 2).
3.
A synchronization detection circuit on the DCS-6A compares the waveform of the internal oscillator
to the waveform on port 2.
4.
When the signals of both sources (actual clock and reference clock sources) are in
synchronization, the requested clock source (port 2) is latched in as the new system clock.
5.
Now port 2 is the new system clock; the internal oscillator is no longer part of the configuration;
and port 4 remains as the alternate clock source.
Содержание Intraplex T1 DCS-9500 CrossConnect System
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