Comtech EF Data CDM-570A - Manual (Page 507)

F–1
Appendix F. CLOCKING MODES
F.1 Overview
When dealing with satellite modems, the subject of clocking can be a complex issue. This
chapter describes the various clocking options that are available with the CDM-570A/570AL.
The CDM-570A/570AL 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.
F.2 Transmit Clocking
There are four transmit clocking modes in the CDM-570A/570AL. Figure F-1 illustrates these
modes. Note that, while EIA-422/449 signal mnemonics are used in this figure, the description
applies equally to V.35 and synchronous EIA-232
F.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) that is 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 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.
G.703 Clock extension: This is a special case of Internal Tx Clock, where the internal clock
generator is perfectly locked to an externally applied G.703 (T1 or E1) signal. See Section F.1 for
detailed information.