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E.2.2
TX Terrestrial
In this mode, the modem expects to see the DTE provide the clock so that it can phase-lock its
internal circuits. In this case, the modem does not provide any signal on ST, but instead requires
a clock signal on TT that is synchronous with the data. If no clock is present, an alarm will be
generated and the modem will substitute its internal clock.
G.703: This is the 'natural' clock mode.
IP: Does not apply.
E.2.3
RX Loop-Timed, RX=TX
In certain circumstances, a terminal at the distant-end of a satellite link may be required to
provide a clock to the DTE equipment that is locked to the receive satellite signal. This is similar
to Internal Clock mode in that the modem will source ST to the DTE, but now the timing is derived
from the demodulator. The DTE then clocks from this source and gives the modem transmit data
(SD) that is synchronous with this clock. It is optional whether the DTE also returns the clock (TT)
– the modem can accept it, if it is present, but uses ST if it is not. If the demodulator loses lock,
the modem's internal clock will be substituted, so an accurate and stable clock is present on ST
rather than a clock that may jitter and wander in a random fashion.
G.703 and IP: Does not apply.
E.2.4
RX Loop-Timed, RX<>TX (Asymmetric Loop Timing)
The CDM-425 Advanced Satellite Modem incorporates circuitry that permits loop timing when the
TX and RX data rates are not the same. In this case, the clock frequency appearing at ST will be
whatever the TX data rate is programmed to, but will be phase-locked to the demodulator's
receive symbol clock. In all other respects, the operation is the same as for 'standard' loop timing.
G.703 and IP: Does not apply.
E.2.5
External TT with ST = RX Satellite
This is a special case that addresses a particular need for backwards compatibility with the
SDM-300A modem. It is, in many ways, similar to the standard TX Terrestrial mode described in
Sect. 9.2.2 in that, in this mode, the signal appearing on TT is used to provide the timing
reference for the transmit side of the CDM-425 Advanced Satellite Modem. However, in this
mode, the ST pins on the interface are active and provide a copy of the RX Satellite clock (i.e.,
the clock from the demodulator, prior to the buffer).
This mode is subject to certain limitations:
•
RX and TX data rates must be identical.
•
No framing (EDMAC, etc.) may be used.
•
RS-422 and V.35are the only interfaces supported and used on the Type 'D' 25-pin
connector.
Appendix E
CDM-425 Advanced Satellite Modem
E–2
Revision 0
MN-CDM-425
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