Comtech EF Data CDM-500 - Manual (Page 56)

Rev. 1.3 7–1
7
Chapter 7.
CLOCKING
When dealing with satellite modems, the subject of clocking can be a complex issue. This
section describes the various clocking options which are available with the CDM-500.
7.1 Transmit Clocking
There are three transmit clocking modes in the CDM-500. EIA-422/449 signal
mnemonics will be used for illustration, but the description applies equally to V.35, and
synchronous EIA-232.
7.1.1 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 if it is present, but uses ST if is not.
7.1.2 External Clock
In this mode, the modem expects to see the DTE provide the clock, so that it can phase-
lock its internal circuits. In this case, the modem does not provide any signal on ST, but
instead requires a clock signal on Terminal Timing (TT), synchronous with the data. If no
clock is present, an alarm will be generated.