1
097-55501-01 Issue 6 – June 2003 27
2
5
3
Description
expansion shelf (or shelves) attached to a master shelf, you require
extended capacity clock cards.
- Nine-input with an ST2E rubidium oscillator (090-55514-02)
- Nine-input with an ST3E quartz oscillator (090-55512-02)
Two clock cards are required for redundancy; oscillator types can be mixed.
Each clock card receives the input reference signals from the Input/Alarm
Connector card, determines their quality for use by the system, and selects
the most appropriate input signal to use as the active reference. During
normal operation, the active clock tracks the active input signal, and
produces a precise frequency for the output driver cards based on the input
reference. The standby clock does the same thing except its output is not
used by the output driver cards. The standby clock backs up the active
clock, ready to become active when necessary.
If the input reference is disqualified, the clock card selects another qualified
input signal as the reference. If no input references are qualified, the clock
card goes into holdover, using its own oscillator as the system reference.
1.3.5.1 Passthrough Mode
The TimeHub 5500 supports a capability to provide a usable clock in a
situation where both clock cards are removed. The capability is called
Passthrough and resides on the TimeHub management card. The
management card has connection to the timing inputs, which it continually
monitors for signal presence.
Passthrough mode occurs in the following instances:
• Passthrough will only be possible when a clock card has qualified
at least one input since the management card was powered. The
management card monitors the clock cards for qualified inputs; if
no qualified input has been detected, Passthrough will not enable
upon removal of clock card clocks. When the management card
has determined that Passthrough is ready, an automatic message,
PTAVAIL, is issued. Passthrough readiness can be checked at any
time via the TL1 RTRV-COND command.
• When Passthrough mode is activated, the input selected as the
reference is based upon the following hierarchy:
- The first choice will be the active reference that had most
recently been selected by the clock card before it was removed.
- If the first choice is not available (due to loss of signal),
Passthrough will attempt to select from the other inputs that
To Next Page
Loading...
Need help?
General
