AT&T MERLIN LEGEND Release 2.0 Equipment And Operations Manual page 76

Hardware Components
Besides the slots, both basic and expansion carriers have a backplane with an
input/output (I/O) bus that interfaces with the modules.
Figure 2-1. Carriers
Processor Module
Placed in slot 00 of the basic carrier, the processor module controls system
features and programming, The main component of the processor module is the
feature module, This component provides all the system's release-specific
capabilities and features. All system programming is stored in the feature
module in non-volatile memory.
The feature module plugs into the main board of the processor module, which
contains the 66000 microprocessor, a built-in 1200 bits-per-second (bps) data
modem, built-in diagnostics, RAM, a real-time clock, and interrupt circuitry, and
interfaces to the other modules through the I/O bus on the carrier backplane.
2-2 Control Unit
BACKBOARD