Comtech EF Data CDM-625A - Manual (Page 592)

CDM-625A Advanced Satellite Modem MN-CDM625A
Appendix E Revision 3
E–2
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 Terminal Timing (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 and ASI: This is the ‘natural’ clock mode.
IP and Audio: 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 Send Timing (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 (Send Data, or SD) that is synchronous with this clock. It is optional
whether the DTE also returns the clock (Terminal Timing, or 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, ASI, IP and Audio: Does not apply.
E.2.4 RX Loop-Timed, RX<>TX (Asymmetric Loop Timing)
The CDM-625A 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, ASI, IP and Audio: 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-625A. 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.