Comtech EF Data SDM-2020 - Manual (Page 178)

SDM-2020 Satellite Demodulator Revision 4
G.703 Data Interface MN/SDM2020D.IOM
11–12
11.1.6 Interface Loop - Through
The interface provides a “Loop-Through” mode of operation. The loop-through mode is
intended to allow synchronization of RX data (RD) to send data (SD) when an
SDM-2020 demodulator and a SDM-2020 modulator are co-located (see Figure 10-3)
Loop-through mode is selectable only when the G.703 interface is installed in an SDM-
2020 modulator.
• Interface Operation Within SDM-2020 Modulator
When loop-through mode is executed (turned on) send data (SD) at the input to
the interface is routed to both the interface receive data (RD) output and the host
modulator data input. This mode of operation does not
interfere with
transmission of data over the satellite link. Transparent to the user, the following
actions occur on the interface:
♦ Recovered clock and data from the send data (SD) input is looped to the
interface RX data path.
♦ Interface RX data rate and line coding are set the same as TX.
♦ The RX Doppler/plesiochronous buffer is set for minimum depth and buffer
reference clock is set to TX Terrestrial.
• Interface Operation Within SDM-2020 Demodulator
Although loop-through mode is not selectable when the G.703 interface is
installed in a SDM-2020 demodulator,
the following items must be considered
when setting up the demodulator.
♦ Connect receive data (RD) output of the co-located modulator to the send
data (SD) of the co-located demodulator.
♦ Set the interface transmit data rate and line coding the same as the co-located
modulator.
♦ Set the G.703 interface buffer clock reference to Transmit Terrestrial.