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Chapter 9. CLOCK MODES AND
9.1
Introduction
When dealing with satellite modems, the subject of clocking can be a complex issue. This section
describes the various clocking options that are available with the CDM-625. There are two
fundamentally different interfaces provided by the modem:
•
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 hence easier to
understand. However, when used with Drop and Insert operation, the subject again
becomes more complex.
9.2
Transmit Clocking
There are five transmit clocking modes in the CDM-625. EIA-422/449 signal mnemonics will be
used for illustration, but the description applies equally to V.35, HSSI and LVDS.
9.2.1 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 the user returns TT to ensure the correct clock/data relationship.
G.703 and ASI: 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 and Audio: This is the only available mode.
DROP AND INSERT (D&I)
9–1
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