Verilink AS2000: The Basics (880-502981-001) Product Manual - Manual (Page 30)

System Information
2-14 Verilink Access System 2000: The Basics
NOTE: Any ACP based AS2000 application module which uses a data
bus to pass data to another module must use shelf timing.
Table 2-7 ACP Timing Options
Timing Source The timing source can be any port on the module, the internal
clock on the module, or an external clock source connected to the
external timing input DIN connector on the module’s rear
connector (CIM).
Shelf Sync Master The shelf sync master is the ACP module responsible for putting
the shelf timing table onto the clock timing bus. The sync master
cannot be an NCM module. If the sync master is removed from the
shelf, any ACP modules relying on the shelf timing table will
momentarily have performance interruptions until a new sync
master is selected.
Timing Definition
Shelf
Timing
Some ACP module within the shelf provides the timing for all
modules set for shelf timing.
Card
Timing
The module uses its internal clock or synchronizes its internal
clock to the signal from the network, application equipment, or
external timing ports. Shelf settings are ignored.