Comtech EF Data CDM-550T - Manual (Page 95)

Chapter 10. CLOCKING MODES
10.1 Overview
When dealing with satellite modems, the subject of clocking can be a complex issue. This section
describes the various clocking options which are available with the CDM-550T.
10.2 Transmit Clocking
There are four transmit clocking modes in the CDM-550T. RS-422/449 signal mnemonics will be
used for illustration, but the description applies equally to V.35, and synchronous RS-232.
10.2.1 Internal Clock
In this mode, the modem, assumed always to be the Digital Circuit Equipment (DCE), supplies
the clock to the Digital Test Equipment (DTE). (The RS-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 if it is present, but uses ST if is not.
10.2.2 External Clock
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.
10.2.3 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 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), synchronous with this clock. It is optional whether
the DTE also returns the clock (Terminal Timing, or TT) - the modem can accept if it is present,
but uses ST if it is not. If the demodulator loses lock, the modem’s internal clock will be
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