24 TimeHub 5500 User’s Guide 097-55501-01 Revision M – January 2009
Chapter 1 Description
Functional Overview
Functional Overview
The TimeHub 5500 accepts up to five incoming reference signals (inputs 0–4). The
inputs can be an analog 10 MHz primary reference source (PRS) and up to four
DS1 reference signals (1.544 Mbps). Using criteria programmed by a system
administrator, the TimeHub selects one of these qualified references as the active
tracked reference. The TimeHub 5500 then filters and distributes precise timing
signals to the office equipment. The TimeHub 5500 can monitor up to five additional
inputs on a second, optional Clock card, but these inputs cannot be selected as the
active reference that drives the outputs.
Each Clock card (CLK1 and CLK2) tracks the input reference sources and
generates a precise and stable 1.544 MHz square wave clock signal. These signals
are sent through the backplane of the TimeHub shelf to the Management Card and
to the Output Driver cards.
The Management Card (MC) contains a clock generator that provides an unfiltered
1.544 MHz square wave to the backplane. This signal, called the pass-through
clock, provides a last-resort clock signal to the shelf if both Clock cards are removed
or if they become inoperative. This feature is available on Master or Remote Master
shelves with Clock and IMC cards equipped with hardware version “B” or higher and
firmware version R4315C or higher. For more information on passthrough, see
Passthrough Mode, on page 35.
Figure 1-3 is a block diagram of the TimeHub 5500 system. It shows how the 1.544
MHz sync signal is distributed from the Clock card(s) and the Management Card to
all the Output Driver cards in the Main shelf and the Expansion shelves.
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