CDM-625 Advanced Satellite Modem
Revision 1
Clock Modes and Drop and Insert (D&I)
MN-0000036 (Ref MN/CDM625.IOM)
9.3
Receive Clocking
There are three receive clocking modes in the CDM-625, plus an additional setting used for Drop
and Insert only – see later section.
9.3.1 Buffer Disabled (RX Satellite)
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. There is still a minimum buffer in use to de-jitter the effects of
removing overhead framing.
G.703:
Applicable.
9.3.2 Buffer Enabled, TX=RX (TX Terrestrial or Int (SCT) Clock)
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 or the internal (SCT) clock. This is a relatively simple case, as the output clock for the
buffer is derived directly from either ST, TT or the external source.
G.703:
Applicable.
9.3.3 Buffer Enabled, RX<>TX (TX Terrestrial or Int (SCT) Clock )
This is an uncommon case, where the receive data rate does not equal the transmit or external
reference. The modem will generate a phase-locked buffer output clock which uses the selected
reference, regardless of its frequency in relation to the receive data rate.
G.703:
Applicable.
9.4
X.21 Notes
For X.21 operation, use the RS-422 pins, but ignore Receive Clock if the Modem is DTE, and
ignore Transmit clocks if the Modem is DCE.
9–3
