Comtech EF Data CDM-570 - Manual (Page 192)

CDM-570/570L Satellite Modem with Optional IP Module Revision 12
Clocking Modes MN/CDM570L.IOM
10–2
10.2 Transmit Clocking
There are four transmit clocking modes in the CDM-570/570L. EIA-422/449 signal mnemonics
will be used for illustration, but the description applies equally to V.35 and synchronous EIA-232.
10.2.1 Internal Clock
In this mode the modem, assumed always to be the DCE, supplies the clock to the DTE. (The
EIA-422/449 name for this signal is Send Timing, or ST.) The DTE then clocks from this source
and gives the modem transmit data (Send Data, or SD), 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. At rates above 2 Mbps, Comtech EF Data highly recommends
that you return TT to ensure the correct clock/data relationship.
G.703: The internal clock mode does not apply; the clock is always recovered from the incoming
signal, and the modem locks its modulator clocks to this.
G.703 Clock extension: This is a special case of Internal Tx Clock, where the internal clock
generator is perfectly locked to an externally applied G.703 (T1 or E1) signal. See Section 10.5
for detailed information.
10.2.2 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, 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.
10.2.3 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 which 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), 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: Does not apply.
10.2.4 Rx Loop-Timed, Rx<>Tx (Asymmetric Loop Timing)
The CDM-570/570L incorporates circuitry which 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