Comtech EF Data SDM-300A, Installation And Operation Manual

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Options SDM-300A Satellite Modem
A–24 Rev. 4
A.1.4.3.3 MUX Operation
The MUX receives terrestrial and ASYNC data from the selected receivers. The
terrestrial data flows into a small First In/First Out (FIFO) buffer. The FIFO buffer aids in
the rate exchange between the terrestrial data rate and the overhead rate. The data can be
clocked into the MUX by the terrestrial clock or an internal clock.
ASYNC data is received by the RX section of a Universal Asynchronous Receiver/
Transmitter (UART) programmed by the M&C for the correct data protocol. The
incoming ASYNC data is sampled with a 16x clock in the middle of the bit time.
AUPC data is received from a serial M&C interface. The overhead clock is generated
from the terrestrial data clock by a phase-locked loop. Inside the multiplexer, overhead
bits (1/16) are added to create a sub-frame, frame, and multi-frame structure. The AUPC
data from the M&C interface and the ASYNC data are inserted into the framing structure.
The framed data is output to the modulator card on the modem at the overhead rate.
A.1.4.3.4 DEMUX Operation
The DEMUX section functions in a “reverse” manner to the MUX side. Data, including
overhead, is received from the Demod 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 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.
A.1.4.3.5 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.