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Theory of Operation
The TX clock phasing choice is made from the front panel in the Interface Configuration
menu.
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Data received by the modem is output on the Receive Data (RD) line, while the recovered
clock is output on the Receive Timing (RT) line. The RD can be set to NORMAL or
INVERT at P4 on the interface card. Receive Clock selections can be made from the
front panel in the Configuration Interface menu or from a remote terminal.
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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. DM can be set to NORMAL
or INVERT at P1 on the interface card.
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. RR can be set to NORMAL
or INVERT at P3 on the interface card.
The RS-232-C interface also provides bi-directional relay loopback of both the clock and
data at the DCE interface. In LOOPBACK:
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Loop timing is supported by the selection of the RX Satellite Clock source in the
Configuration Interface menu of the front panel for "TX Clock Source." When RX
Satellite is selected, the transmit clock (ST) is replaced by the clock recovered from the
satellite (RT).
4–24
The AUTO setting is used when there is no jitter on the clock source.
The NORMAL setting is used when standard clock and data relationships exist.
The INVERT mode is used when the incoming clock is inverted from the
standard clock and data relationship.
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.
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.
SDM-100A Satellite Modem
Rev. 0
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