ComStream CM601, Installation And Operation Manual

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Theory of Operation 2-7
7. The demodulator function mixes the IF carrier down to baseband. The
demodulated channel symbols are then:
a. Decoded by a Viterbi or Sequential decoder depending on the installed
modem card and the ordered options.
b. Differentially decoded
c. Descrambled
8. The data is directed across the backplane through any optional, receive data-
processing card (for example, the SCC card).
9. The data is directed back to the modem card where it is converted to the
appropriate electrical levels and then passed to the user.
10. The receive clock and data are provided by the demodulator card to the data
interface card across the CM601 backplane.
Data I/O Interface
The modem card provides aV.35 or RS-449 data path for data sent to and from the
modem. Other interface types are available using the option card slot.
Doppler Buffer
ComStream’s Doppler Buffer is a feature that is standard on every CM601 modem
card. It buffers data received from the satellite on a first-in-first-out (FIFO) basis.
Data is input to the buffer using the receive clock from the satellite signal. The
Doppler buffer is of programmable depth to ensure that the receive clock and data
from the satellite are synchronized or are set to another clock as necessary.
M&C Card
The M&C card, illustrated in Figure 2-6, performs traffic management and relays
messages between the front panel or remote control interface and the target
CM601 card. In this bus-oriented control configuration, the M&C card is called
the master, and all other cards are called slaves.
Figure 2-6 M&C Card
G328-01
RS-232 or
RS-485
remote control
Status relay
Relay sense lines
Front panel
interface
Backplane
control bus
System reset
20-MHz
reference
20-MHz
TCXO
Master
Microcontroller
Battery-Backed
RAM and Real-
Time Clock