Premier Mounts CDM-600 사용자 설명서

10–1 

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-600. 
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. 

There are five transmit clocking modes in the CDM-600. EIA-422/449 signal mnemonics 
will be used for illustration, but the description applies equally to V.35, and synchronous 
EIA-232. 

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: 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.