Comtech EF Data SNM-1001 - Manual (Page 40)

SNM-1001 Network Control Modem Theory of Operation
Rev. 1 4–9
Path memory is 80 states in depth. The path with the maximum metric is designated as
the survivor path, and its data are used for output. The difference between the minimum
and the maximum path metrics is used as the means of determining synchronization of
the decoder.
The output data may then be descrambled and differentially decoded. Both of these
processes are optional, and may be selected locally or remotely. The data signal out of
the differential decoder is sent to the interface card for formatting and output.
The synchronization signal is used for lock detect, and is sent to the M&C. The raw BER
count is generated from re-encoding the decoded data and comparing it to a delayed
version of the input data. The count is then sent to the M&C for further processing.
4.3 Monitor and Control
The modem uses a sophisticated micro-controller module to perform the M&C functions
of the modem. This module is located on the demodulator board, and is referred to as the
“M&C.” The M&C monitors the modem and provides configuration updates to other
modules within the modem when necessary.
The modem configuration parameters are maintained in battery-backed RAM. The RAM
provides for total recovery after a power-down situation.
Extensive fault monitoring and status gathering are provided.
All modem functions are accessible through a remote communications interface.
4.3.1 Non-Volatile Memory
Non-volatile memory on the M&C module allows it to retain configuration information
without prime power for at least one year. Should the modem be powered down, the
following sequence is carried out when power is re-applied to the M&C:
1. The micro-controller checks the non-volatile memory RAM to see if valid data
has been retained. If valid data has been retained, the modem is reconfigured to
that information.
2. If non-volatile memory fails the valid data test, a default configuration from
ROM is loaded into the system.