Polycom RealPresence RMX 4000 Hardware Manual page 51

RealPresence Collaboration Server (RMX) 4000 Components and Components Replacement
ISDN Clock Source
Each RTM ISDN card has its own primary and secondary clock source. The first span to synchronize
becomes the primary clock source and the second span to synchronize becomes the secondary clock
source. This clock is used to synchronize ISDN spans only (it is not the system clock).
A single clock source triggers an alarm that can be turned off by setting the appropriate flag in the system
configuration.
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