Comtech EF Data SDM-2020 - Manual (Page 179)

SDM-2020 Satellite Demodulator Revision 4
G.703 Data Interface MN/SDM2020D.IOM
11–13
Modulator
SD
RD
Demodulator
SD
RD
Tx IF
Rx IF
Figure 11-3. Co-Located Modulator and Demodulator
11.1.7 Test Modes
11.1.7.1 Interface Loopback
The unit provides an “Interface-Loopback” test mode of operation. When loopback 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.
Send data (SD) is routed through as much of the interface circuitry as possible before
being returned to the receive data (RD) output. This mode of operation does not disrupt
transmission of data over the satellite link when installed in an SDM-2020 modulator.