Comtech EF Data CDM-570A - Manual (Page 508)

CDM-570A/570AL Satellite Modem with Optional Packet Processor MN-CDM570A
Appendix F Revision 2
F–2
F.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: This is the ‘natural’ clock mode.
F.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: Does not apply.
F.2.4 RX Loop-Timed, RX<>TX (Asymmetric Loop Timing)
The CDM-570A/570AL 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: Does not apply.