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Chapter 10. CLOCK MODES AND
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-600L.
There are two fundamentally different interfaces provided by the modem:
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10.1
Transmit Clocking
There are five transmit clocking modes in the CDM-600L. RS-422/449 signal mnemonics
will be used for illustration, but the description applies equally to V.35, and synchronous
RS-232.
10.1.1
Internal Clock
In this mode, the modem, assumed always to be the DCE, supplies the clock to the 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 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: 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.
DROP AND INSERT (D&I)
Synchronous clock and data interfaces (RS-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.
10–1
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