Mocomtech CDM-570 - Manual (Page 213)

Chapter 9. CLOCKING MODES
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-570L.
There are two fundamentally different interfaces provided by the modem plus the
optional IP Module Ethernet interface:
• Synchronous clock and data interfaces (EIA-422, V.35, etc.) that permit great
flexibility concerning the source and direction of clocks. These are complex.
• G.703 interfaces combine clock and data into a single signal (and are referred to as
self-clocking). In their basic form these are less flexible and easier to understand.
• For the optional IP Module Ethernet interface, clocking is internally controlled and
clock selection is not available.
9.1 Transmit Clocking
There are four transmit clocking modes in the CDM-570L. EIA-422/449 signal
mnemonics will be used for illustration, but the description applies equally to V.35, and
synchronous EIA-232.
9.1.1 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: 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.
9–1