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Theory of Operation
Data received by the modem is output on the Receive Data (RD) lines, while the
recovered clock is output on the Receive Timing (RT) lines.
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•
The Request to Send (RTS) line is hard-wired to the Clear to Send (CTS) line by JP11,
pins 5 and 6, on the Demodulator/M&C card (AS/4973), since the modem does not
support polled operation.
Data Mode (DM) indicates that the modem is powered up. Receiver Ready (RR)
indicates that an RF carrier is being received and demodulated with a sufficiently low
error rate for the decoder to remain locked.
The RS-422 interface also provides bi-directional relay loopback of both the clock and
data at the DCE interface. In LOOPBACK:
•
•
Loop timing is supported by selection of RX Satellite Clock source in the front panel
"INTERFACE CONFIGURATION" menu for "TX Clock Source." When RX Satellite is
selected, transmit clock (ST) is replaced by the clock recovered from the satellite (RT).
Three fault outputs are provided on dry contact Form-C relays:
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•
•
They are available on the FAULT connector on the rear panel of the modem.
Generation of these fault conditions is described in Chapter 3.
4–16
Receive Clock NORMAL mode should be selected for applications that require
the rising edge of the clock to occur in the middle of the data bit time.
INVERT mode puts the falling edge of RT in the middle of the data bit.
Note: This selection can be made from the front panel in the Configuration menu
or from a remote terminal.
From the DTE side, SD is connected to RD, and either ST or TT (in INTERNAL
or EXTERNAL mode) is looped back to RT.
From the modem side, the received data and recovered clock are routed back to
the modulator input for retransmission.
COMMON EQUIPMENT
MODULATOR
DEMODULATOR
SDM-100A Satellite Modem
Rev. 0
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