Comtech EF Data CDM-570 - Manual (Page 193)

CDM-570/570L Satellite Modem with Optional IP Module Revision 12
Clocking Modes MN/CDM570L.IOM
10–3
data rate is programmed to, but phase-locked to the demodulator’s receive symbol clock. In all
other respects the operation is the same as for ‘standard’ loop timing.
G.703: Does not apply.
10.3 Receive Clocking
There are three receive clocking modes in the CDM-570/570L, plus an additional setting used for
Drop and Insert only – refer to Figure 10-1 for details.
10.3.1 Buffer Disabled (Rx Satellite)
When the buffer is disabled, the receive clock (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.
10.3.2 Buffer Enabled, Tx=Rx
In this instance, it is required that the buffer be enabled, so that the clock and data appearing on
RT and RD are synchronous with the transmit clock. This is a relatively simple case, as the output
clock for the buffer is derived directly from ST, TT or the external source.
G.703: Applicable.
10.3.3 Buffer Enabled, Rx<>Tx
This is an uncommon case, where the receive data rate does not equal the transmit clock. 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.
10.4 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.