Comtech EF Data SDM-100A - Manual (Page 187)

SDM-100A Satellite Modem Options
Rev. 0 B–5
B.1.1.4 Demultiplexer Operation
The operation of the DEMUX section is similar to the operation of the MUX side.
Data, including overhead, is received from the Demodulator card in the modem at the
overhead rate.
The DEMUX locates the framing in the overhead and locks to the frame sync pattern
generated by the multiplexer on the transmitting end. Once locked to the framing, the
terrestrial data is clocked into the Doppler buffer with the overhead clock and an enable
line.
The ASYNC channel data is stripped out of the frame structure, and is buffered up in the
TX portion of a UART. The UART then transmits the data with the selected protocol to
the appropriate drivers to the end user. The AUPC data is also stripped from the frame
structure and is sent to the M&C via a serial interface.
B.1.1.5 Buffer Operation
The buffer has two serial interfaces to the M&C interface. The first serial interface is
used to download the desired buffer size. The second serial interface is used to provide
the M&C with the information necessary to calculate the fill status of the buffer. Three
discrete lines are provided:
• One line to center the buffer on command.
• Two lines to indicate either an overflow or underflow condition.
The Doppler buffer receives data clocked by the overhead clock from the Demodulator
and an enable line from the DEMUX. The data is stored in RAM. Four options are
allowed to clock the data out of the buffer:
• TX
• RX
• Internal
• External
Based on this selection, terrestrial data is clocked out of the buffer to the selected drivers
and on to the end user.