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Appendix E. CLOCK MODES
E.1
Overview
When dealing with satellite modems, the subject of clocking can be a complex issue. This
appendix describes the various clocking options that are available with the CDM-425 Advanced
Satellite Modem.
The CDM-425 Advanced Satellite Modem provides two fundamentally different interfaces:
•
Synchronous clock and data interfaces (EIA-422, V.35, etc.) that permit great flexibility
concerning the source and direction of clocks. These cause the most confusion.
•
G.703 interfaces, in which the clock and data are combined into a single signal (and are
referred to as self-clocking). In their basic form, these are less flexible and therefore
easier to understand. However, when used with Drop and Insert operation, the subject
again becomes more complex.
E.2
Transmit Clocking
There are five transmit clocking modes in the CDM-425 Advanced Satellite Modem. EIA-422/449
signal mnemonics will be used for illustration, but the description applies equally to V.35, HSSI,
and LVDS. Figure E-1 provides a graphic depiction of these modes.
E.2.1
Internal Clock
In this mode, the modem, assumed always to be the Data Circuit-termination Equipment (DCE),
supplies the clock to the Data Terminal Equipment (DTE). (The EIA-422/449 name for this signal
is Send Timing (ST).) The DTE then clocks from this source, and gives the modem transmit data
(Send Data (SD)) that is synchronous with this clock. It is optional whether the DTE also returns
the clock (Terminal Timing (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.
IP: This is the only available mode.
Appendix E
DROP
AND
E–1
CDM-425 Advanced Satellite Modem
INSERT (D&I)
Revision 0
AND
MN-CDM-425
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