Comtech EF Data CDM-550T - Manual (Page 96)

CDM-550T Satellite Modem Revision 3
Clocking Modes MN/CDM550T.IOM
substituted, so an accurate and stable clock is present on ST, rather than a clock which may jitter
and wander in a random fashion.
10.2.4 Loop-Timed, RX<>TX (Asymmetric Loop Timing)
The CDM-550T incorporates circuitry which permits loop timing when the TX and RX data rates
are not the same. In this case the clock frequency appearing at ST will be whatever the TX 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.
10.3 Receive Clocking
There are three receive clocking modes in the CDM-550T:
10.3.1 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).
10.3.2 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.
10.3.3 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.
10.4 X.21 Notes
For X.21 operation, use the RS422 pins, but ignore Receive Clock if the Modem is DTE, and
ignore Transmit clocks if the Modem is DCE.
10.5 Loop Timing with Sync RS-232
The CDM-550T distinguishes between synchronous and asynchronous RS-232 by detecting clock
activity on the TX Clock pin of the interface. If no clock is detected, it is assumed that the mode
is asynchronous. Therefore, if loop timing is employed in a synchronous RS-232 application, it is
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