Comtech EF Data CDM-500 - Manual (Page 57)

Clocking CDM-500 Closed Network Satellite Modem
7–2 Rev. 1.3
7.1.3 Loop-Timed
In certain circumstances, a terminal at the distant-end of a satellite link may be required
to provide a clock to the DTE equipment which is locked to the receive satellite signal.
This is similar to Internal Clock mode, in that the modem will source Send Timing (ST)
to the DTE, but now the timing is derived from the demodulator. 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 it is
not. If the demodulator loses lock, the modem’s internal clock will be
substituted, so an accurate and stable clock is present on ST, rather than
a clock which may jitter and wander in a random fashion.
7.2 Receive Clocking
Like the transmit side, there are three receive clocking modes in the CDM-500.
7.2.1 Buffer Disabled
When the buffer is disabled, the receive clock (Receive Timing, or RT) is derived directly
from the demodulator, and hence will be subject to plesiochronous and Doppler offsets.
In certain instances, this may be acceptable, and in the case of Loop Timing, it is
essential that the buffer is disabled (which is done automatically).
7.2.2 Buffer Enabled, Rx=Tx
In this instance, it is required that buffer be enabled, so that the clock and data appearing
on Receive Timing and Receive Data (RT and RD respectively) are synchronous with the
transmit clock. This is a relatively simple case, as the output clock for the buffer is
derived directly from either ST or TT.
7.2.3 Buffer Enabled, Rx<>Tx
This is an uncommon case, where the receive and transmit data rates are not equal. The
modem will generate a phase-locked buffer output clock which uses the transmit clock,
regardless of its frequency in relation to the receive data rate.
7.3 X.21 Notes
For X.21 operation, use the EIA-422 pins, but ignore Receive Clock if the Modem is
DTE, and ignore Transmit clocks if the Modem is DCE.