Comtech EF Data CDM-600/600L - Manual (Page 157)

CDM-600/600L Open Network Satellite Modem Revision 3
Clocking Modes and Drop and Insert (D&I) MN/CDM600L.IOM
8–3
8.3 Receive Clocking
There are three receive clocking modes in the CDM-600/600L, plus an additional setting used for
Drop and Insert only – refer to Figure 8-1 for details.
8.3.1 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.
8.3.2 Buffer Enabled, Tx=Rx (Tx Terrestrial or External Clock)
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 either ST, TT or the external source.
G.703: Applicable.
8.3.3 Buffer Enabled, Rx<>Tx (Tx Terrestrial or External 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.
8.4 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.