Comtech EF Data CDM-625, Инструкция по установке и эксплуатации

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CDM-625 Advanced Satellite Modem Revision 1
Clock Modes and Drop and Insert (D&I) MN-0000036 (Ref MN/CDM625.IOM)
9.2.2 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), 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.
9.2.3 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 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 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.
9.2.4 Rx Loop-Timed, RX<>TX (Asymmetric Loop Timing)
The CDM-625 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 data
rate is programmed to, but 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.
9.2.5 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-625. 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
• No RS coding may be used
• Only these interfaces supported on the 25-pin Type ‘D’ connector may be used:
o
RS-422, V.35, HSSI, and LVDS.
9–2